So finally my new 80w 24V heater cartridge arrived and yeah, zero issues anymore. I can turn the cooling to full blast and the temperature will hold steady.
This really made me realise that the stock cartridge was just completely underpowered, I'm printing with a 0.6mm Volcano CHT nozzle and before it could just barely keep up with PLA temperatures. I never even considered that it could be too weak...
Well, thanks for the help. Now I just have a bunch of other issues to address with this printer...