Some thoughts:
Sabotage targets
It sounds like there's going to be agents working against the Soviet faction. I've seen this done poorly where the Soviet soldiers would simply be targets to pick off by sniping or backstabs, which destroys roleplay. In one particular case there was a journalist character that wanted to visit the base, but due to the lack of trust they were turned away, so in a culture where players exist to be killed you'll find potential roleplay opportunities being dismissed.
How do you promote the idea of players trusting eachother over a deathmatch culture? You could start off with a rule that protects interactions on a certain level. You can substitute the target of the Soviet soldier by offering other valid targets like unattended vehicles, power generators, ammunition dumps, all the way down to minor sabotage targets like tyres, jerry cans, or even just glass windows. If a rule existed that you couldn't kill a player without reasonable provocation, then you could still disrupt them in other ways.
The Soviet faction might not actually need jerry can props, but the point is that on a roleplay level the Soviet soldiers should act like they need those jerry cans. As a tool for roleplay, citizens that take part in low level sabotage shouldn't be summarily executed. The Soviets are there probably hoping to have some local citizens tolerate them, and in turn that might mean being somewhat lenient, making the need for arrests and trials over just killing any citizen that hints at standing up to them.
Lore broken down into events
Something that seemed to be a popular concept was the idea of breaking down lore-influencing events into even smaller events. For example:
The arrest of a local militant leads to information that he was connected to a defector. A search of the militants house reveals that the defector will visit a certain landmark at a certain time, and a scouting mission reveals that frag grenades would be a good method of assaulting the position from a ridge in a surprise attack. After some grenade training, they assault the point and capture the defector. The defector is processed by military police and put before a firing squad. Keeping all characters informed of each development can help immerse them in the storyline.
Quality
It's not really anyone's place to keep everyone else happy, but I do want to float this idea of helping prevent key people from burning out. People that had unique skills and motivations, that added a level of quality to the scene. Making them feel appreciated and supported could benefit the experience on the server in the long run. There's a lot of good people that left at some point, perhaps with valid grievances, and nobody could replicate what they did (they way they were doing it). Maybe we don't even need to solve all our problems to keep people happy, we just need to be able to vent about them...
Mono-culture
I don't really know how to articulate this point, but it's the sense that there are a mixture of different experiences you can have on a server like this, and different people lean towards different parts of that experience, so letting people do their own thing - focus on what they're good at, and not constrict them means we can have a range of different ideas working alongside eachother. Servers that were more monotone seemed to evaporate after a short time.
Rank tenture
A faction like the idea of a Soviet army stationed at an FOB is going to need officers for the sake of roleplay structure. If officers are getting murdered somehow (which I think is likely to happen in this lore) then to maintain the roleplay structure we'd need to see several officer characters. The idea of rank tenure here would be that if an officer is killed then to fill the void they left they can make a new character of a similar rank to the one they lost, although as a different personality.
Too many times I logged on to a roleplay server the day after an officer character was lost, to find that there were too few to actually run the thing.
Language
Characters encountering language barriers can be a mess. Can they all share a common language so that they can all potentially communicate with eachother? If my character encounters a language barrier then it might be realistic, but I quickly start to struggle with ideas for improv roleplay if I have no idea what to do. I'm just suggesting that idea in terms of keeping things running smoothly. I have no idea how popular/unpopular it would actually be. If you needed an interpreter character and there wasn't one available then you might both be standing around awkwardly.