You can either change it (not always that hard even with minimal soldering skills, it’s actually harder to find a decent battery), or use it plugged in. My wife’s old Kindle Fire is my permanently plugged-in, wall-mounted kitchen recipe book.
My problem with repurposing an old Android device is the security issues. Firmware doesn't get updated anymore. An alternative OS like LineageOS or PostmarketOS takes care of most of the software issues, but not the firmware itself.
Another issue is that the battery is going to be years old, and on 'recent old' Android devices its much more difficult to remove the battery.
But many/most of these older devices discuss, do not - and my personal anecdote is that of the tens of phones+laptops I have owned myself and managed for family through the years - there were 4 cases of bulging: 3 were constantly-charging devices (as in, 3-5 years with hardly any time not charging), and one that was dropped from height and bulged immediately.