Phones that can have their battery removed tend to work fine plugged in with their battery removed, so that's an option. Also a good reason to make sure you don't buy a device that you can't open. I made that mistake with my current phone, and I definitely won't be making that mistake again if I get another smart phone.

And phone batteries turn quickly into spicy pillows if they are kept constantly attached to a charger.

I guess you increase the cost of your setup by adding an intermittent power switch / timer that turns charger power on and off. With MQTT, the server itself could drive the switch, and turn the charger on when the battery is low enough, turn it off when full enough.

If you don't mind disassembling and finding connectors or soldering, you can connect a power supply with the same voltage as the battery to the battery connector on the main board.

I've read that leaving phones constantly in the charger is a fire hazard. It's at leat worth checking it out before trying this at home.

Interesting, I’ll need to see how this affects my use case. I assumed that devices plugged into power operate on power while charging the battery, at least my laptop does.

Some phones have limiters to keep the battery at 60%-80%.

I believe most can do this with the right software.

It's not a fix, but it should extend the life considerably.

If the phone is old enough, you can just pop the battery out.

I've tried a few and so far it worked for all of them.