S.T.A.L.K.E.R Anamoly is free and a great single player fps with a great vibe to it. Doesn’t quite fit your criteria though as I wouldn’t say there is much of a campaign. Very interesting dynamic world and AI system though.
Edit. Highly recommend the GAMMA version. Get installation files @ their discord.
Check what speed your mobo is capable of with the nvme drive option. A blazing fast nvme drive is a relatively cheap way of getting some more response out of your PC. That and a newer GPU it’ll feel like a whole new computer.