As a long-term casual player, you probably know all about the units and what they can do.
But you probably have bad eco management. I didn't bother to learn eco management when I was a casual. That's the first area to work on if you want to get your skill/rating up. The guide from FtX is designed to educate you on that.
If your goal is to keep improving beyond that, so you can be a team game beast, you can learn a lot by playing 1v1s, because you have to take responsibility for all aspects of the game. In team games, other people will scout for you, they will overflow you power, they will send units to save you sometimes, etc. In 1v1s, everything that happens is basically up to you, so you learn much more quickly where your deficiencies are. If someone is going to coach you, the fastest way for them to assess your abilities (so they can decide what to teach you next) is to watch your 1v1 replays.
So even if you prefer team games, the path to enlightenment is through the ladder. To get good at team games, you should play 1v1s (to become well-rounded) and you have to play team games (because team games have different dynamics from 1v1s).
Some high-rated players might say "I never needed to play 1v1s to get good at team games." But ask yourself: is your experience similar to their experience? Did you just effortlessly git gud? Just because they can be well-rounded, high-skill players without playing 1v1s doesn't mean you will.
I don't think you need to wait for your skill to plateau before you ask people to look at some of your replays and give feedback. I think it's a bad idea to wait until you get frustrated and hit a wall, before you get feedback from good players. You also need to think for yourself (not all advice you get from others will be good advice) and you need to analyze your own replays if you want to improve.
If you're interested in a longer written guide, I wrote one that is focused on 1v1s: https://forums.faforever.com/viewtopic.php?f=62&t=17559