But a vehicle that new would be pretty far outside the budget right now.

40kWh of house batteries is probably going to run you closer than you'd think to the price of one of these cars... unless you can find some second hand ones (all the same type so you can string them together).

Anyway, you’ve got me a bit curious now about the possibility of finding an EV battery in a junkyard and maybe using that as solar battery storage. Though … those usually run at pretty high voltages.

They often come in multiple cells, depending on your abilities you might be able to wire them up to keep the voltage low. One thing to consider is that your ability to monitor the battery's charge level and your power usage, in my experience, came as part of the battery. If you're going to be living off grid, it might be important to have that functionality which wouldn't come with random ex-car battery cells.

And there’s always the backup generator, which will cost a fortune to run, but will prevent anybody freezing.

If you're gonna burn stuff inside, maybe a (modern) fireplace would help? Cost to run depends a lot on where you can get wood from, but it doesn't need power, they generally put out a lot of heat, and you can even get wet-back varieties to help with hot water heating. That would go a long way to getting you less reliant on solar generation in the winter months.

Don’t worry about that, they’re plastic drums. The biggest concern would be that they might at some point freeze and burst … but that would never happen if I fill them with an antifreeze solution. The real biggest concern is just how much space they would take up … and possibly how much weight they’d be adding to the floors.

Plastic cracks with age too! Especially with 200kg of water inside them for a long period. The weight on the floor could be a concern, water is very heavy.

Yeah, may end up having multiple charge controllers in addition to the inverter’s built-in solar charge controller(s).

Personally, I’d prefer to have those systems be entirely separate in the first place, so each one can be sized according to need and each one can be independently replaced if it breaks. But it seems that most inverters capable of handling our ~6.2kw absolute peak usage already come as ‘all in one’ inverters with solar charge controllers built in … I guess I might as well use the built-in charge controller, at least for part of the solar array.

While we miss out on a bit of power when our generation is over 10kW, I don't think it's that much, and we have a 15kW system. Realistically making 10kWh over a big portion of the day is way more important than making 15kWh briefly in the middle of the day. You might need that time in winter though, depending on daylight hours. But personally I'd think more panels to get generation up over a long period would be more important than capturing the peak generation.

Our solar company used this site for their estimates, https://www.opensolar.com/ which you might be able to sign up for and see estimates of what you'd need in your location.

6.2kW isn't actually that hard to hit. You might use that if you oven is heating up while you use the microwave. Well, maybe this is harder to hit if you have 120v power, but with out 240v power it's pretty easy. I presume you're talking kW (power load) not kWh (energy usage over time).

Even if we changed absolutely nothing about our usage, if we built the system to be capable of providing 4kw reliably, we’d only need to use the generator ~6 hours per year.

Your battery can likely handle the peak loads above this. I'd expect you to break out the generator only when batteries are running flat, not to handle peak loads. A battery will probably provide 10kW peak load or a bit above this. Your main issue will be if you don't manage to charge your batteries during the day due to poor weather, then you need to run 2 (or 3) days on your batteries instead of 1.