this is very good news. I had tried Nextcloud several years ago as a self host and it seemed quite clunky and unfinished. I guess things have moved on if its ready for org use!

this sounds doable. i do something similar with syncthing. it depends on the internet connection speeds. see how it goes with wireguard. between two home NAT networks you might find something lile tailscale easier to set up. wouldnt recommend doing this when youre not able to drive over there and fix things occasionally. ethernet preferable for reliable connection over wifi at the remote end. if you secure the pi well and update it it should be fine. note that you probably need to trust the remote household with your data.

not a web dev. If I did want to make some minimal fast loading pages that don’t look awful, is there a decent framework out there that will do the styling? A bit of html is fine but learning CSS is just not on my to do list.

most solar systems will not operate in a power cut.

edit: system detects no grid supply and shuts itself down. other systems with an off-grid mode exist, just theyre not the default and usually cheapest option a lot of people round here install.