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
For some reason I tend to break screens way before the battery dies usually. I am not afraid to replace a battery or even a screen if phone is not totally out of date. Having said that there seems to always be a point when they start acting semi eratically or stop booting.
I had a totally outdated and probably insecure Samsung S6 edge 2 years ago that started rebooting randomly.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.
That is a common issue on old hardware. You have to be wary because some like android devices get their truststore updated regularly but it doesn't stay in rom so if you do a complete device reset you get back to the original ones that are now useless to connect to app stores and uodate servers.
Having replaced batteries on my pixel... The best trick ive found is a ziplock baggie full of near boiling water plopped on the screen until the glue loosens.
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.
So happy being able to buy a Fairphone. Have replaced the screen once, the battery twice, and the back twice too. Fairphone 4 on the its 5th year. I want to upgrade but this thing is really resilient.
I had a totally outdated and probably insecure Samsung S6 edge 2 years ago that started rebooting randomly.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.