A new survey from mobile operator Vodafone UK has claimed that people are now keeping their mobile devices for longer, with the majority upgrading every 4 years
Just get a the battery replaced. With the new rule for the EU forcing companies to make the phones with user replaceable batteries, it'll be even easier.
I thought about it last time but the whole thing where I'm not getting OS updates anymore make me anxious. I'm not sure that's actually a problem though.
Sorta. (Android is technically Linux) What you would want to look into is a custom Android ROM. Something like OmniROM or ArrowOS.
As a side note though, installing can be a bit technical for some people and some apps don't like to work outside of the "security" of a factory version of Android. If you are interested, I would browse around the custom Android communities here (if there are any yet) and check out sites like https://www.xda-developers.com/
Rooting is a slightly separate thing from sideloading a custom rom, rooting gives apps you give permission to access to system files whereas sideloading a rom replaces the system files but doesn't necessarily give you root access. Both involve a similar process of unlocking the bootloader, installing a custom recovery and flashing a file so it is easy to get the two things mixed up.
No, there are unfortunately no truly workable Linux OSs for phones yet AFAIK. But there are plenty of Android-based OSs that work really well, and are usually maintained for quite a while, depending on the popularity of your device. Look into LineageOS, or Pixel Experience, to name a few.