Which instance you are on makes some difference but not a lot (in my opinion) if you're on an instance with a lot of like-minded people you'll see their posts on your local stream, and it's more likely that you'll see more like-minded posts in your "federated" stream. So it does give you a bit of a boost in terms of seeing posts you might like. But not as much as following the group and the hashtag, and gradually building up a good home stream by following people you enjoy reading.
I used to be on some Reddit autism groups but I found those quite fighty so not good for me. r/autisminwomen and r/autism were the main ones I used to visit.
I wouldn't worry too much about changing your current account (if you mean your past posts?) just start as you mean to go on, if that makes sense.