I just wanted to add I have similar concerns. Offgrid, independent cameras:
The RLC-410/510W refurbed reolinks are about $33/45 rn on their ebay site.
Those, and several other PoE type cameras, have a 12v power port. You can do as probably someone already said: get a $20 solar panel battery trickle charger w integrated charge controller (usually built onto the back), an LFPo or even lead acid AGM battery, and the 410/510 kits come with everything you need, in combination with the panel kit.
Build the whole thing into a bird feeder etc, and put the 5.8ghz signal within range of your wifi. You'll need a 64 or 128GB high endurance microsd card for another $10-16.