Mechanicus is very good. It has a flaw of getting easier as you progress, and events not being consistent, but as far as TBS' go it's very good. While not guardsmen, it is possible to build around faceless infantry troops.
Fire Warrior is ...not good, but guard feature heavily as enemies.
Now for some games I've not played, but heard good things about.
There is supposedly an Open Xcom mod that is really good, and I think you can play as guard in it?
I liked Space Marine, but never played Space Marine 2.
Boltgun got hype then seemingly vanished.
There's a sisters of battle game for VR (Battle Sister) that is supposedly average to above average for a VR shooter.
For a guard-like game. Xcom, Halo Wars, Earth Defense Force, and Helldivers might fit the bill. They all focus on "guys with guns vs xenos horrors" to varying degrees. Helldivers and EDF are more comedic action games, Halo Wars is a RTS, and Xcom is a TBS.