David,
Thanks for the update, your system looks very nice.
In your boots, I think the next thing I'd consider is water. I'm on a well, too, and being without water during a power outage is a bummer.
If you've got room for an approx 40 gallon barrel of water in that basement, you could use a small 12VDC RV diaphragm pump to take water from that barrel and pressurize your home water system. Presto: Hot water for (short) showers, water at the kitchen sink, flushable toilets, the whole shebang.
Water barrel:
44 Gallon Brute container ($52). Approved by NSF for food contact. I'm not sure about possible PFAS contamination during fabrication, so I wouldn't drink from it without running the water through a carbon filter, but it should be fine for bathing, etc. Every 6 months I'd pump out the water, add fresh water, and put in a few capfulls of fresh bleach.
Water Pump: This
SeaFlo 33 series pump costs about $50, gets good reviews, puts out about 1.6 GPM at 35 PSI, turns on at 35 PSI, turns off at 45 PSI. Tech specs
here. It draws about 6A at 12VDC, and you could hook it straight to your battery (so no inverter standby or conversion losses. You can flush your toilets without making a trip to the basement to turn on the inverter). A 1.3 GPM shower ain't a firehose, but it works and lots of RVers get by with less.
For a "hook it up when I need it" case, I'd just rig up 1/2" tubing from the pump to a convenient hose outlet in the basement. Your existing well pressure tank serves as a pressure reservoir. That should be okay as long as your water flow at a tap/shower doesn't exceed your pump capacity for very long. You'll hardly notice the electrical draw on your battery (you'll drain the entire water barrel in about 30 minutes of pumping = 3 AH (36WH)).
Barrel, pump, fittings, etc: It would probably cost about $130 in parts, and might get you by for a day or two of judicious water use. At any rate, when on a private well it's handy to have some water on hand. You don't need any fittings at the water barrel side, the input to the pump can just be draped over the top of the barrel and a weight on the end will keep it at the bottom. I >think< the pump will self prime.
(Pro tip: When you first turn on the shower/sink and are waiting for the warm water to arrive, catch the water in a bucket. Dump that bucket back into your barrel or use it to flush a toilet, etc)
Mark