For devices with bad batteries that aren’t easily replaceable, I just keep them plugged in and either use them like I would a desktop or use it to host stuff. Most laptops will work with the battery removed so long as they are plugged in.
For a phone you still might be able to host stuff, but you could also use it as a primarily storage device.
If they’re Pixel phones GraphneneOS is insanely easy to flash and is the one of most secure ROMs out there. I’ve heard good things about CalyxOS too.
A lot of people speak well of LineageOS, but I have no idea how involved the installation process is. Another option that I’ve heard of is postmarketOS but that’s a lot more complicated to install.
Heh, heh. I didn’t know that people actually bought the books on kindles. It’s ridiculously easy to get any book you want for free on them.