Take a look at online UPS systems. They convert incoming power to battery voltage, keep the battery charged, and output via an inverter that's made to run without being tied to the grid. They shouldn't be that costly if you can find them without large batteries, instead just hooking them up to your own.

Transfer switch to a very small inverter run off a battery; add your loads one-by-one and you should be fine during a prolonged outage (and as long as there is sunshine...)

can you directly use the DC as a reserve source? like, install car plugs in the house? there are lots of smaller everyday electrical gear for this standard

It's actually a much more interesting question than you might expect. Lots of modern appliances could run perfectly on DC of around the RMS voltage. For example the most modern fridges rectify the incoming mains to drive a brushless DC motor, this allows them to run at an appropriate speed rather than using bang bang control. Even an an induction hob, you might think it was most reliant on having an AC waveform but actually they need a much higher frequency so again they rectify the incoming mains.

Obviously it's not quite that easy, for example an oven uses resistive heating elements which would work fine on DC, however, the relay in the control circuitry wouldn't be able to switch a DC voltage.

That works for small cabins and lighting, as a 12v car outlet requires the same thickness of wire as a normal AC outlet. This is somewhat popular to do if you don’t need to use any high power appliances as then you can skip needing an inverter.