Since the Wolfenstein games were already mentioned: The Saboteur

It aged quite a bit, but there are some nice mods to make it look and handle a bit better on Nexus Mods.

Tactical Breach Wizards

It is similar to Into the Breach in some ways.

Last time I played CSS was in 2018, so I can’t really answer that. But according to SteamDB there are currently 11000 players.

I have a lot more hours on Counter-Strike Source than that, but that was before Steam started counting hours. I also played a ton of Sims 2 and 3.

A ton of Darktide.

Death Stranding