Usually I replace my phone when the battery is almost dead and a full charge doesn't last more than a few hours. At that point the phone is usable but not good enough to sell. I usually keep it as an emergency backup phone
I had a totally outdated and probably insecure Samsung S6 edge 2 years ago that started rebooting randomly. I think that prior to that it was so old I could only install stuff from f-droid and aurora because roots certs installed on that old android version were too old for playstore to work. Too bad as you can't easily find smartphones in similar sizes anymore.
Knocking on wood my pixel 6a still going strong after 2 years.
Did that for 20 € for my Oppo Find X3 Pro at my closest tech store and saved me from buying a brand new phone
Use a thin tool to break the seal all the way around then peek under it like youre checking for spiders. Feels like most electronics these days have sneaky weak ribbon cables with ultra brittle connectors behind them.
This might be what all the youtube tutorials recommend as well I cant recall where I saw it. Of course the YouTube teardowns also show you in advance where the booby traps are.