I'm particularly fond of the 30+0 time control for new-ish players, but it's probably one of those things: ask 100 people and get 100 different answers.
The difficulty with long games online is people who stall, abort, cheat, things like this. Ideally you can find a local club and play long games there OTB (OTB = over the board, as in offline games), or join an online group where people are more trustworthy.
Long games are useful because they give you enough time to build good habits and try to break old ones. In speed games you only have enough time to play the way you always do.
If you're interested, I made a topic about the type of habits I consider most important:
https://www.chess.com/forum/view/for-beginners/the-most-important-concept-for-all-beginners
You can (more or less) get that from the forums. Post questions and also games.
The best kind of game to post is one that lasted ~30 moves (or longer). Preferably a game where you lost (or were outplayed / losing at some point). Make the title something like "help me understand this move/game" and people will respond. I (and many others) participate in this kind of casual coaching.
Some ubiquitous advice:
- Play long time controls
- Solve tactics puzzles
- Study books (topics like strategy and endgames)
- Openings are useful, but it's common to spend too much time on them
- It's common to spend too much time on speed games