I've been playing full ring $3 normal speed NLHE SNG on ACR and having fun with it as a change from another grind I have been working on. When I get serious (obsessed) I will certainly do the study and reading. But for now its just funsies and I would appreciate just some basic input from you guys/gals.

Most players in these seem too nitty, which says alot since I am a nit in full ring cash. Is overfolding a common leak down here? If so that is tragic, to let yourself dwindle to nothing.

Bet sizing. I am careful with this and it seems like it befuddles the average opponent. Three streets of value with tptk and half pot 3 barrels, none of the other players play this way. When I bluff-catch (or pay it off) on the river they are betting 1/4 pot or overbetting pot.

All in pre. I am running fine and always have at least 40 percent equity, usually a lot more and I am rarely all in. Is this normal or just run-good?

20BB with 5 players left, this will happen quite often and I always open up here yet they stay the same or even tighten up.

ICM. I have some feel for this but not the concrete understanding as pertains to STT. When a player is under ICM stress, the gap between what they can open raise vs call with widens dramatically, right? I know more to it, but that seems to be the most of it.

Around 20BB or lower you have a reshove stack so you should open a bit less but 3-bet all-in more than you would do a normal 3-bet not-all-in if you were deeper.

Yes ICM generally means you can call less. It also should mean you can open less but often that's balanced other people being able to call less. It's very spot specific and also the correct answers are not fully known (as how an outcome sets you up for the next hand, and the one after that etc. is also important).