MacBook Pro M2Pro Clamshell > Eternal Monitor > NO POWER

Leave the computer plugged in. Leaving it plugged in will not damage the computer or the battery. "Clamshell" mode has always required that the computer be plugged in.

Change Battery settings on a Mac laptop - Apple Support

Change Energy Saver settings on a Mac desktop computer - Apple Support

Adamnater wrote:

See, I had always understood that leaving it plugged in when it's fully charged will harm battery health over time.

Your understanding is incorrect. The MacOS manages the power/charging system very well. There is no need for you to try to micromanage the system. In fact, leaving the computer plugged in reduces the rate of accumulation of battery cycles.

Your Mac is not a cheap garbage toy, that charges itself to oblivion.

Your MacBook Pro  uses ‘smart charging’ to charge in the optimum way, and only when necessary. Plugged in is Not necessarily actively charging. There is substantial hardware and software cooperating on battery and charging issues.

Connected power sources simply 'offer' their charging ability, and the computer decides what to accept and when. Simply asserting a charging voltage against one of the interface(s) will NOT successfully charge your MacBook Pro.

Any external power supply that provides "USB Power Delivery" (including certain Docks and Displays) must negotiate over the USB cable using USB Power Delivery Protocol, and can not 'force itself' on your Mac. The Voltage and Current are delivered only after your Mac requests and the charger agrees to supply power under certain controlled conditions. The computer is in control of the entire process.

Your Mac will NEVER overcharge.

if you are any MagSafe adapter, the LED in the end of the cord is illuminated under program control...

...by command sent by the computer,

The cord from the power adapter body has only TWO wires in it, power and return.