I appreciate the desire to only back up critical loads in a new sub panel (SP). More efficient.
However, customer wants at least 20 kwh of battery, and everything, including heat pump HVAC, backed up.
Really my reason is: I HATE the "spaghetti" ((#10 and #12 wires and wire nuts) and ton of labor involved with pulling and rebuilding a new back up SP. So many issues arise, starting with 3-wire circuits.
I have learned to put that money toward more battery and just backing up the whole damn house - if possible.