That meter main buried in the wall, posses some real challenges.
1. I don't think (can't remember) that you can leave that enclosure with unprotected service conductors, and return from another source.
2. Buried in the wall, makes it very hard to access.
I'd be curious what is on the opposite side of that wall. In my case (45yr old SoCal house), the opposite side of my in-wall, exterior flush-ish mount meter and main load center, is the garage. As such, running protected service conductors not all that difficult (even easier in my case as unfinished garage wall (not involved to pull off plywood and unpainted drywall)... I'm planning hybrid inverter and whole house batteries on other side of garage wall from meter/main panel...so real easy to access, even though in-wall
Then, my question is whether there is a retrofit kit for than main bus bar?
I'm with Tim, with breaker interlock seeming to make more sense at this point, presuming one will work in that main load center??
The only point is to be able to power house if inverter fails, right? so does NOT need to be anything fancy. Grid will be up, house will not have power, someone will go look and see Hybrid Inverter off (or smoking ;^), or whatever) and you simply need a way to bypass inverter.
Though you will need a rapid shut-off capability (breaker in outdoor panel suffices, right? did for me and my inspection). So, if Inverter in the garage (better than outdoors), you need to provide an externally accessible, clearly labeled shutdown switch for PV. Doesn't the EG4 now have a remote shutdown switch? or did I read about that, but not yet available?
And something to consider is desired cost vs reliability (of Solar PV and/or battery working if/when grid-forming hybrid inverter fails)? Whole other discussion, but I went micro-inverter so that PV production not as vulnerable, and chimney in middle of roof with PV panels, so really needed to anyway.
And looks you could combine some of those 240A breakers (with dual throw, quads, which is whayt I ended up needing to do and passed city inspection... ) if you needed to free up some breaker space. and you actual usage low-enough to not create a safety hazard (even if against best practice)...
...following shared for context, and in case sparks thought for OP...
Also, not sure relevant, but as I also have only a 100A service/main breaker, an important factor is that my busbar is rated for 125A, which allowed me to add 19*425W panels without getting a service upgrade, or moving loads to a sub-panel. I'm still looking for move most circuits to a sub-panel, inside the garage by door into house for easier access... My main load center is larger than OPs, but full with double breakers... yea, I know. but in reality, pre PHEV, my max consumption was in the 4kW range, ever. With a PHEV charging at 16A, max went closer to 7kW. With new heat pump A/C (post solar install), max grid pull has been 8kW... so yea, not good to have breakers adding up to more than 3X+ main breaker size... but reality is that is legacy wiring, and I could easily consolidate a lot of those circuits (ex 15A circuits largely for lighting)... and will when moved to new sub-panel.
I see no reason for a critical loads panel, per se, for me. First, I'm on a non-curtailment circuit from PoCo, and grid outages are _really_ rare. My usage isn't high, even with A/C running, so why bother with 'critical' loads... I don't have any 'vampire' draw that would materially impact battery life in an extended grid-down situation (other than refrigerator, which is primary item to power when grid-down). My large loads (PHEV charging, A/C) are all directly controllable. I don't need something 'automatic' to prevent someone else at home from doing something stupid (thankfully). And one can set up home automation sometimes much cheaper than re-wiring.
My plan/primary reason for a sub-panel is to get breakers easier to access and free up main breaker space.
I do have 1 kitchen 15A breaker that trips easily with current microwave and anything else significant like electric kettle, garbage disposal, or popcorn maker).. but rewiring kitchen on 1st floor of 2-story house on apposite side from meter/main load center, won't be cheap/easy. Presuming at some future date, natural gas appliances will be hard to come by, I'm assuming I'll eventually need to run a new 50A circuit for electric oven and stove.. and I'll bring in a spare 20A circuit while I'm at it. Until then, not a big deal, other than having to do outside and across house to flip breaker.
Again, sub-panel plan has nothing to do with 'critical loads', which with as few breakers as OP, the chance 'critical' and non-critical loads on same circuit seems high. so why bother? I've read those with a well-pump that they don't want running if grid-down, makes sense, or old AC units and old thermostats. Though some automation and a smart switch/breaker might be way cheaper?
_Only_ once we need 50A circuit to Kitchen, or add a high Amp EVSE, would I need to look into a PoCo service upgrade (which is underground in my neighborhood, means digging up driveway, etc. so NOT looking forward to it). So getting busbars, SER and other wiring capable of 200A would be good for future proofing, but unless major load additions planned for near-term, seems wasteful to buy 200A breakers, switches at this point (unless price premium really minimal, and usually isn't). Better to wire for 200A, but wait until needed to get actual breakers/switches capable of 200A... as relatively easy to swap out??
