I have played since about 2010 and joined FAF on another account back in about 2012. I played one game back then but my godawful internet meant that I lagged out. Since then I have played on and off but only with friends against bots.
The few people that I have introduced to this game, had never played an RTS before. So I went through the FA Campaign with them. Not going fast or anything just doing the usual shit like stalling the expansion until you have 50 soul rippers, Playing Meltdown as a super turtle holding the mesa with 500 ravagers etc. (By the way having timed expansion on by default is a BAD idea. If you are only new and fumbling around having your problems get worse and getting destroyed in the campaign is only going to make you feel further from the people in the casts you watch and join because of.) Getting them to understand how to play at the most basic level. If you have ever seen someone who never played an FPS as a kid try one, look it up, you'll see them unable to move and shoot at the same time and other basic tasks.
For people who join solo and don't get this I can understand why they'd leave. I think a tutorial where the player is hand held through a generally good build queue would be good, and another that removes building and just supplies them with units to micro. Another good level might be to cover the slight faction Pros and cons to show strats they might not know, same for units.
Adding a Trainer mode might be another idea, a teamed observer who can ping things and manage a timed expansion with the other trainer to have a managed 1v1 with the other Trainee. Maybe even break these down to land only then add air then navy. You could even run Trainer tournaments with brand new players.
Gamifying is another option, add exp. Give players a few exp for every game they stay till the end of. This adds a level separate to your rank, maybe even have them "pick" a faction (does not effect gameplay just like a kind of a selectable badge) and the faction with the most exp after a week gets a cool chat color or something. Obviously take a bit of time avoid exp grinding, maybe only MM, no cheat campaign victories and trainer matches give exp. When people finish a campaign game have a message that says xxxx did that mission 3 min faster and show them their time rank.
Clean up the mod vault. Get all the big mods to have pictures and proper descriptions, make it so that if a mod has a dependency it is automatically added or at least warms the player that they will need it. Maybe don't have the same mod twice of 2 different versions in recommended mods. Another thing might be to pick a big player or popular community member and have a combined X's recommended Ui mods that is a one click add.