America's army still has a decent amount of people playing. Mostly older folks, but there are tons of community made maps. Give it a try.

It's old, but SWAT 4 is still the best tactical shooter ever made IMO.

Mods like Elite Force make the experience even better.

Planetside 2 is sort of a MMO Tactical fps. Involves (or can involve if you find the right group) a decent amount of geographical planning with squads and platoons across each playable continent. Very large worlds. Armor vehicles and aircraft are available too to add a new layer of logistics.

However it's plagued with performance and bad UI issues due to the 14 year old Forgelight engine, and the dev team is pretty unorganized these days so the player experience is not terribly great, especially for someone starting out fresh on the game that doesn't understand its many, many quirks (and does not have the skill to go up against the 1000+ hour gaming chair sweats that almost exclusively populate the PS2 servers now). That doesn't mean it's not worth trying though- it's free to play, so give it a try, get into a platoon, and see how you like it.

I mean, CS:GO runs smoothly on Linux, and afaik so do Arma and Siege (not sure on the last one). They're not open source, but yeah, they run.

Are you specifically looking for shooters that run on Linux? Escape from Tarkov and BattleBit are probably my two favorite tactical shooters at the moment, but Tarkov definitely doesn't run on Linux and I'm unsure about battlebit.