Just because he considered the debt repaid doesn’t mean she felt the same way. Even with the debt repaid, she’s still there because he saved her life.
When Boba first asks her to help him recover his ship, she says, "If I help you, my debt is paid."
After they recover the ship, Boba tells her that her debt has been repaid and asks where he can drop her off. She says she'll go along for the ride.
Then chronologically later on, when they meet up with Mando on Tython, she tells him she's with Boba because she owes him a life debt.
Sure, maybe she decided that just helping Boba recover
Slave 1 wasn't good enough.
As an aside, rewatching bits of episode 4, I think I've figured out what Boba's angle is: after they kill the sarlacc and are sitting by a campfire, they share this conversation:
Fennec: "Are you serious about forming your own house?"
Boba: "How many times have you been hired to do a job that was avoidable? If they only took the time to think, how much money could have been made? How many lives could have been saved?"
Fennec: "Then you and I would be out of work."
Boba: "I'm tired of our kind dying because of the idiocy of others. We're smarter than them. It's time we took our shot."
I think the idea is that he's annoyed that working for Jabba nearly got him killed, and he doesn't want to work for anyone else anymore. He wants to be his own man, working for himself, choosing his own jobs ... a simple man making his way through the galaxy.
I'm guessing he chose to take over Jabba's territory because it was convenient. Bib Fortuna was easy to get rid of. Tatooine is familiar territory for Boba. Maybe, once he's gotten himself established, he intends to make things better for other Tusken tribes.
One thing they didn't address in the show is why he tells Fennec that Bib Fortuna betrayed him. What was that about?
