I first joined FAF like many people at the beginning of the first lockdown in March, but really started playing just a couple of months ago so my new player experience is still relatively fresh.
That being said, I am at a 1.3k global ranking now and do intend to stick around for a while, so I guess this is the opinion of someone who got over the "new player hump".
First and foremost, I agree with many of the posts here: The game is indeed very hard, unintuitive and complex. More casual game modes that can bridge the gap between the (coop) campaign and multiplayer would also be greatly appreciated to onboard new players.
But really, what got me closest to quitting was the toxic community at ranks below ~800. At those ranks, every game is a complete clusterfuck, and sometimes (often) one side just straight up gets destroyed by the other, even if both sides were of the same skill level. Imo there is nothing much that can be done about that. The game's economy of unrestricted exponential growth means that even a 20% difference in skill, can result in you having 10 times the number of units than your opponent just a couple minutes later. So new players will regularly get destroyed at lower ranks and there is nothing we can do about it.
But what we can change is how they FEEL about getting destroyed! Every such game that ended with people starting to flame and then ctrl+k-ing their entire base left a sour taste in my mouth while games where I got equally murdered that ended with friendly advice on what I could have done better, made me want to play again right away!
It's not even about people like me I am concerned about. I have thousands of hours of competitive multiplayer games under my belt by now, so I am (sadly) very used to online toxicity by now. But, the toxicity at lower ranks does prevent me personally from introducing some of my more casual, less abuse proof friends to the multiplayer side of FAF. Because who wants to introduce their friend to the multiplayer experience of a game they adore, just for them to be called slurs for minutes on end?
Playing 2v2 with them is only a partial solution as the scale that many casual players love is found much easier in 4v4 or larger multiplayer matches. There are many other good things to be said about larger team games too:
On many noob-friendly maps, the initial minutes are conflict free, which just feels much more relaxed than the "action from second 1" that smaller maps have.
Larger team games usually have dedicated roles for each spot. While suboptimal for learning FAF the most "efficient" way, one single role like "you are the air player, just have more planes than your opponent" is conceptually much easier to understand than the myriad of stuff you have to do in 1v1 and 2v2 games.
Larger team sizes also make it easier to write off losses as not entirely your fault. It's not that I'm preaching unaccountability here, but loosing 5 times in a row, and knowing for certain it's your fault, can be a much harsher experience than just knowing you could have done a bit more.
None of this is new information of course. I mean, there is a reason that lower ranked players get drawn to 6v6 dualgap and not 2v2 Fields of Isis.
So the most natural place to onboard new players is in large team games - at least half of which end in flaming at lower ranks...
My suggestions therefore would be a Karma system of sorts, fully built into the client.
Make flaming, griefing, ctrl+k-ing your base reportable and start restricting people's functionality once peoples karma score drops too low.
Maybe start by restricting all-chat during games, so they at least can't flame the other side for "being smurfs" or whatever, maybe even disable team-chat too if they drop too low. Put a skull picture next to their name in the lobby so people know right away not to take them seriously, ban them for a day or a week, stuff like that.
You could reward positive behavior too, although I don't know what those rewards could be yet.
tl;dr: Have been playing FAF for about 5 months, the toxic community at lower ranks is imo the biggest hindrance to introducing new players to the multiplayer. My solution would be a Karma system to punish bad behavior.