A good TA will do as much or as little as the client needs, tbh. I'm a veteran Disney visitor who is an A type planner that enjoys spreadsheets and the like, but I still use a TA largely to handle the administrative stuff. When all this COVID stuff was going down for example I didn't have to wait hours on the phone with Disney to cancel any of the things I had to call for and couldn't do online, our TA took care of it.
You'll see a lot of folks mention that they don't use TAs because they like to be "in control of the reservation" but the truth is with a decent TA that's not an issue. Ours is extremely prompt about returning emails. All she really does 99% of the time is take payments and give them to Disney, once in a while she will book things for me that I don't have time to take care of (like cabanas or spa appointments), but mainly I like having another set of eyes on my trips. Like when special offers and discounts become available, I don't have to be up at 4am, or whatnot, by the time I'm awake and checking my email with bleary eyes, she already applied the discount and I see an updated reservation.