Each panel has a busbar with some rating. We aren't allowed to run more current through it than the rating. Putting a backfed breaker at far end of the panel means current from PV is flowing the opposite direction as current from main breaker, so it subtracts rather than adds. However, the authors of NEC were afraid in the future someone might relocate the backfed breaker adjacent to main breaker, resulting in the currents adding. So, they put a limit on it, total 120% of busbar rating. That might get hotter, probably not cause a fire. Without this concession, if a 100% limit, most couldn't add any PV.
Decide which panel you want to feed PV into. Determine its busbar rating, and main breaker rating (you might substitute a smaller main breaker if that helps.) Main breaker rating + PV breaker rating can't be more than 1.2x busbar rating. Same math for every panel up the line including main panel.
I think you have two panels (at first I thought a 3rd). You could always change shed panel or branch to two panels there, so it isn't the limiter.
Key question is what busbar rating your "200A" main panel has. Mine is Square-D QO, and busbar is 225A. 225A x 1.2 = 270, so with 200A main breaker, I can have a 70A breaker feeding PV, or feeding another panel that has PV and some loads.
You can probably change the 100A breaker to either 70A (if 225A busbars in main panel), or 40A. That's a bit low for the shop tools; I use 50A breaker for my buzz box. You may be able to reduce main panel breaker from 200A to 150A (of course need utility to disconnect power while you do.)
But wait - meter panel and 200A main panel? Is there a 200A breaker in the separate meter panel? If so, that's great. That's what I have. If so, put a 200A breaker in the main panel and separate 100A fused disconnect feeding the shed. Then it is pretty much unlimited. (the 200A breaker in main panel is to prevent 200A from grid and 100A from fused disconnect together feeding 300A into main panel.)