CoreXY for plasma table.

For something small it should be OK, but for anything bigger the belts would have to be way to long and to droopy and vibrate like crazy.
Personally, i have built over 20 Plasma and over 10 of 3D printers (mostly Voron 2.4), i would not even bother with it.

That would be around 5 meters of belt for each motor, so 20mm belt might be still OK
If you feel like it, go for it for sure.
And to further give you hope, a friend i taught about building CNC's did build a router with 2x1 meter envelope, using 10mm GT2 belts and it works nicely once he got a hang of feeds and speeds, been in use for over a year, and yes it is CoreXY driven by Marlin and LightBurn... Yeah, don't ask
Somewhere in my process of teaching him, he went a bit sideways!

I'm not familiar with compiling custom kinematics.

There already is a corexy kinematics installed called 'corexykins'

As an alternative there also is a corexy hal component that lets you use trivkins:
forum.linuxcnc.org/10-advanced-configura...kins?start=10#203954