So if anybody wants a solution they won't have to scour the internet for what are essentially the same uses over and over again
Here's a short list I made through searching this sub and other threads+articles:
#WARNING: Keeping your phone constantly plugged into the charger for long periods of time may cause the battery to swell up and potentially explode
#WARNING: Keeping the screen on constantly on a phone with an OLED screen will likely cause burn-in eventually
Keep it as a backup phone
Dedicated camera
Dedicated music player (especially LG phones with qDAC)
Baby/pet monitor
Use it as a smart doorbell
Use it to record POVs akin to GoPros
Smart Home controller
Bedside alarm clock
Universal remote
Remote PC controller
Help science by lending your phones power for research
Dedicated eReader
Dedicated gaming machine
Webcam
Dedicated car phone for GPS etc
Dashcam
Digital photo frame
Education tool for kids
Video call device
Wifi extender/hotspot
Host a media server on it (Plex etc)
Host a normal server on it (c. /u/3Werve)
Timelapse camera
Give it to kids
PC Statistics (CPU, GPU usage)
Wireless adapter for wired controllers
Kitchen recipe guide
PC Monitor
Macro pad for PC
White noise machine
Disassemble and make an art piece out of it
Experimentation (e.g trying out apps or ROMs)
Dedicated chat device (Whatsapp, Telegram etc) (c. /u/utakatikmobil)
Cast Receiver (c. /u/Substantial_Berry_14)
m3u IPTV player (c. Substantial_Berry_14)
###No uses fit you?
Donate it
Recycle it
Sell it
I may link to examples or guides but for now im a bit too lazy. Googling should be enough, if not add site:reddit.com to the end of your query
I haven't had any luck running any of the phones I've had open like that (a Redmi 9T and a few Sony phones), but it's worth a try.
You can probably solder the wires from a suitable power adapter to the internal battery connector pins if your phone still uses those, though you may also need a resistor in-line somewhere to trick it into thinking there's a healthy battery installed.
Make sure you get a proper adapter for the task if you go down this route. Some people recommend just chopping up a phone charger, but putting 5V on a circuit that expects ~4.3V at max sounds like a bad idea to me.