I've been back about 1 year after being gone for about 10 years myself. I poured through the forums like I was a brand new player and that helped quite a bit. In a general sense, the day-to-day game play is roughly the same as I remember, so I more skimmed those topics and read more in depth on other things, especially scouting efficiently. The Discord has been helpful to keep up with more of a current pulse than the forums provide.
By far my favorite change is the every 3 hour recruiting cycles are gone! That's really what caused me to leave a decade ago. It's amazing how much more life flexibility is possible with 6 hour recruiting cycles.
I originally intended to camp out in D3 when I came back, but quickly learned that wasn't going to be my preferred way to play. Of course there are lots of ways to recruit at D3, but it seems that most of the best scout/recruit the D1 pool entirely, or almost entirely. Since the players in the D1 pool can't sign with D3 until the last 24 hours of RS2, that was a deal breaker for me. I have 3 kids, and there's a decent chance they distract me and I miss some recruiting cycles here and there and having to be focused so much on that very end and keep plugging away until the very end just didn't appeal to me. There's a chance I try D3 by only scouting/recruiting the D2 pool since that opens up signings at the very beginning of RS2, which is same for D2 teams scouting/recruiting D1 pool. Because of all that, I think as of now I'm a D2 player, but when I jumped back into the only world that I had previous experience all of the sudden my alma mater opened up so I'm giving high D1 a try even though I intended to go to D2 in that world also.
With all that being said, I'm happy to try to help if you'd like to send me a site mail. You're likely to find other more successful players that have been around more recently, but I'll share what I can.