There's a big red disclaimer on this page that says the feature list is a bit wishful, and the latest version is only 0.1.0. The docs (man pages, start with man aerc) only document implemented features, you may want to start there.

Here's the ticket for maildir support:

https://todo.sr.ht/~sircmpwn/aerc2/16

Several people have expressed interest in working on this, I expect we'll see it for 0.2.0.

As far as configurability is concerned, yeah, the plan is to continue making it more customizable and configurable. Stuff like filters already afford you a lot of options today. At some point I might also be interested in adding a fifo you can use to feed commands into it from a script, too, for turing-complete customization.