More spaces
This section is corrupted because simply "following it and declaring Mission Accomplished" would result in a dangerous configuration.
The question is "how do you get more breaker spaces in the outdoor trailer/ranch panel?" And the answer is have the surge suppressor and 30A load share the 30A breaker. Surge suppressors are not loads and do not need dedicated breakers. If the Siemens breaker is marked for 2 wires per terminal, there ya go, otherwise pigtail off a #6 pigtail. (#6 just to stop people from whining about restricted current path to a surge, if that's not you, #10). If even that is intolerable, get a Siemens 30/50 quadplex such as Q23050CT2 so you can direct wire to the breaker. This "trailer/ranch panel" is an 8/16 and can accommodate all double-stuff breakers, however none exceed 50A.
I don't know the stab limits for a Siemens panel but if you don't know, 125A is a safe assumption. Be careful what you put across from the 100A breaker. The well pump is 1HP so nowhere near 25A. The surge is 0A. So those are perfect. The 20A singles could go across from the 100A also.
Or you could move the surge suppressor to the 40-space panel. You've placed it as close as possible to the utility on the logic that "the outside utility is the only source of surges and that none of my appliances generate surges!" I would place it in the 40-space panel to more directly suppress locally generated surges (looking at you, air conditioner). As regards outside surges, the extra attenuation down the cable run from trailer/ranch panel to 40-space will make it easier for the surge suppressor and wear it down less.
This house looks bonkers overloaded
I see a normal house panel in that Square D 24-space... OK... looks like wild guess 70A to me give or take. Okay.
But then, I see another 40-space panel with no less than ten 240V breakers. We've got 60A to the above subpanel (that doesn't help your Load Calculation any). TWO "furnace" breakers totaling 90A. A 50A A/C breaker. A 50A spa panel. 40A "COOP TOOP" whatever that is. On top of all the normal loads + a well pump. This house does not have gas, does it?
This is way more stuff than you can get on a 200A panel, unless some EMS is happening off screen.
Looking back at the work done in the trailer/ranch panel, I suspect this house has enjoyed many upgrades since the last time an inspector saw it.
- I see a 30A circuit wired with orange NM-B cable (which is not allowed outside).
- I see 200A utility merging with 100A? 130A? of solar to total 300A on that bus going onto the 200A rated bottom lugs and 4/0 feeder to the 200A bussed Square D 40-space. So 300A could show up on the bus of the 40-space. Does it have a main breaker? It really needs one to prevent that from happening.
- And now you want to add 150A more load onto the outdoor Trailer/Ranch Panel. The 120% rule is not enough, and it doesn't even work on this panel, because the loads (note thru lugs) are not between main and solar breaker.
- And EVs are a thing and probably coming to this home. The layout of these panels makes an EVEMS solution challenging.
Now a 150A panel in the shop doesn't necessarily mean 150A of additional load, but absolutely no way. This house is overloaded already, or quite close to it. And Unless someone can show a NEC 220.82 Load Calculation to the contrary, I say this house is already overloaded, and the solar is dangerously misconfigured.
The X-factor: Home battery and/or V2X
One emerging technology is home batteries. The system does "Grid Forming", meaning it can activate your solar panels even though the grid is down. You probably well know that bog-standard UL 1741 grid-tie / grid-following / microinverter panels are simply stone dead when the grid is down, and cannot be used in a traditional DC/battery system.
Modern grid-forming inverters are designed to emulate the grid so accurately that UL 1741 solar panels recognize it as the grid and power up. The battery (must) absorb the solar energy (or the solar won't be fooled). That means the solar doesn't need to be connected at all to the battery system, it just needs to be on the same side of the isolation switch. (There must be an isolation switch so the inverter isn't backfeeding the grid and killing linemen, and the switch has an auxiliary contact that tells the inverter "we are isolated").
So a Grid Forming Inverter + Battery is what the PowerWall and competitors are. And the battery is the expensive part: hold that thought.
Now that they are selling millions of 75 kilowatt-hour battery packs on wheels, with self-propulsion ability, California is insisting that automakers add the scrap of software code required for them to function as home batteries. The most immediate benefit is a solar/battery system can now be installed without having to pay for the battery, which is the most expensive part of it. A knock-on benefit will be the opportunity for the consumer to arbitrage "peak hour" vs "midnight/morning solar" electricity".
All that to say, it might be worth caring about how to set up solar/battery or V2X for grid-down reliability. The pinch point there is the isolation switch.
Option 1: 400A service, but V2X/off-grid solar-battery would be costly
That's pretty straightforward. Get a 400A service configured to be "solar ready" -- i.e. it has terminals on the utility side of the main breakers for the solar to attach. So that removes solar from the equation entirely.
Since you have a trailer/ranch panel already, I would recommend a 400A ranch panel - which is configured exactly like this one, except with a second 200A breaker above the existing one which does not feed the mini-panel. So the mini-panel is hung off one of the 200A mains. So the second one feeds the 40-space Square D panel, which hopefully helps with its load calculation.
Into the "mini-panel" section go the breakers that are already here, minus the solar. The thru lugs off the mini-panel go to your new "150A" shop, except the wire will be 200A because it's easier to enlarge the wire than it is to screw around with the gigantic and very costly 150A branch circuit breaker. Plus obviously you picked 150A because you were scrimping to save money and would rather have 200A at the shop.
Now if you want grid-down solar/battery, again you have to get the solar on the same side of the isolation switch as the battery. That means the isolation switch needs to be either above the meter, or above the 400A "trailer panel". It also needs to be a 400A isolation switch.
The alternative, that's not really practical with your loads, is to have one 200A "side" contain all solar and critical loads, and have the other 200A side simply lose power when the grid goes down. That saves money on the isolation switch, since you can use commodity isolation switches.
Option 2: Put your loads on a diet.
This isn't about deprivation; it's about triumph of technology.
The other solution here is a radical effort to reduce loads in the house. That can take a number of forms. Right off the bat, radically attack HVAC loads first by aggressively insulating to bring the house as close to LEED as possible, and second by getting rid of that 90A of furnace and replacing it with the most efficient heat pumps. That will also cross off Air Conditioning since heat pumps provide that.
Some appliances can be attacked with efficient appliances, such as heat pump dryer (automatic win, since it doesn't heat or freeze the laundry room, and importantly, eliminates the dryer vent and thus a huge air exchange problem). And a heat pump water heater, which has complications, since it chills the utility room. The well-heeled can get on a waiting list for a battery range that plugs into a 120V outlet.
EMS systems can deployed to take advantage of the fact that most loads operate on a "duty cycle", cycling on/off at intervals. Currently, that duty cycle is decided entirely by 5 cent bimetal strips, and you must provision electric service for the worst condition that the bimetal strip gods might throw at you. Add a tiny bit of intelligence to it, and now you don't.
If you can whittle it down enough, you can probably get the house and shop on the 200A.
And the "diet" will help with solar/battery/V2X, because it will keep everything on the same side of a cheaper isolation switch. And for that matter, greatly reduce the burden the battery must carry.
Also don't forget that other battery in a well-insulated house.
I recently built a 30x50 shop with a small living area built out inside of it. Now I need to feed power to it.
I have a main 200amp feed through panel just below the meter. It's a Siemens PW0816B1200TC. Right now that panel is pretty full, see picture attached. I could remove the 2- 20 amp breakers on the left as those feed receptacles right next to the panel.
Ideally I'd like 150amp service running to the shop which is 100' away. For that I'll run 2/0 aluminum cables.
Can anyone guide me through how I can get this done. 
