USB network cards are even cheaper than PCIE if you don’t mind lower performance (if you don’t have USB3 ports you’re limited to theoretical 480Mbit)

Xonotic (arena shooter), 0 A.D. and Beyond All Reason (RTS), OpenTTD and OpenRCT2 (management sim), VVVVVV (platformer).

These are the one I’ve tried, out of all of them VVVVVV is probably the most polished since it was a commercial project that later got open sourced. BAR is pretty good too if you like over the top RTS games. TTD and RCT2 are reimplementations of the original games. I don’t have strong feelings towards Xonotic but arena shooters aren’t my thing so 🤷‍♂️

Don’t forget about PCIe expansion. Just yesterday I got a FireWire PCIe card for 20€ to transfer old DV tapes to digital with no quality loss. Plug the card in and you’re done. To get the same result on a laptop I’d need a Thunderbolt port and two adapters, one of which isn’t manufactured anymore and goes for 150€+ on secondhand stores.

PS. I would remove “CPU heating” from your system if I were you :)