Red Faction: Guerilla, is a third person shooter about a man radicalised into joining a communist revolution after a corporate drone strike kills his brother.

I don't know if it's the sort of game you're looking for but it might be worth giving it a bash.

There is Syrian Warfare. https://store.steampowered.com/app/485980/Syrian_Warfare/

There is also Gesta Final which is a Cuban video game, you will have to pirate it and maybe it will be easier to run it with Proton than with Windows.

https://www.ecured.cu/Gesta_Final

And there is also a Famicom Japanese game called Gebara where you play as el Che and Castro, but you need to play the Japanese version since the Usonian was censored and they changed the story.

Edit: I just noticed you said FPS, but well, hope you like them anyway

Does Red Faction count?

Good ol' Rising Storm 2

I guess there's this

not exactly militia, but you can play the best army on enlisted, make that ppsh go brrrrrrrrrrrr

squad allows you to play as local insurgent militias against the US or UK in a very tactical setting with a huge map. you can strap IEDs to vehicles and do all sorts of cool stuff. the game also allows you to play as the PLA and PLA marines sometimes, which is also super cool.

homefront revolution

Insurgency is a fun multiplayer one, I haven’t played the newer version a ton but it feels like a bit more realistic battlefield while being a bit more arcade-y than other sim type FPS.

Insurgency 2 let's you be a sort of vague rebel and dress them up. Good game and the cosmetics aren't complete ass

it's probably not what you're looking for but Thatcher's Techbase

Arma 3, with mods you can make any scenario you want pretty much and there are a number of scenarios made by community which center around guerrilla warfare.