Bill Self is clearly the best coach ever at KU. He's the only KU coach to win multiple titles (I don't count the pre NCAA titles, which would give Allen 3 instead of 1). He has dominated the Big 12 for basically 20 years now. He's clearly the best coach in school history, and that was actually the case before the championship this year.
With the retirements of Wright and Coach K, Self is also the best active coach in the country. He's ahead of Calipari and Few, while Izzo and Pitino seem to both be on the decline, and those that are rising (Scott Drew, etc.) haven't caught him.
For CBB history, he's a historically great coach. Multiple titles seals that. I don't think he has yet entered the top 5 - Wooden, Dean Smith, Coach K, are definitely ahead of him, and when you start to think of Rupp, Pitino, Calhoun, Iba, etc., he just hasn't entered that space yet. He could with a third title, at which point he passes some of the old timers (Rupp, Iba, Smith). But that's still yet to be done.
Does he still have weaknesses? Yes. His tendency to play for the highest floor, rather than the highest ceiling could burn him in future years. This year there was no great team in college basketball. KU was the best team, and they won it, but this team isn't going to be a squad that, year's from now, is nationally remembered as being "great" like the 2009 UNC team, or the 2012 Kentucky squad, or the 1992 Duke team, or the undefeated squads from UCLA and Indiana in the 1960s and 1970s. Self wants his team to be good every year (part of the reason I doubt he ever has a truly bad season). He coaches them to win 20-22 games even if he passes on the chance to win 30 games by playing a higher ceiling/lower floor player.
Self has become more adaptive, and seems to have loosened up a bit the last few years. I think missing the 2020 opportunity changed things for him. I think he may have felt pressure to win the title that year, but missing out on that, and seeing everything that has happened in the world since may have changed his views and allowed him to relax a bit and let the players play. Remy Martin is a player that would have shrunk under the old Self, with Self dampening his creativity. He thrived in the biggest moments under the new Self, and that's part of why KU won the title.
KU will continue to win 25 or more games every year for at least the next few years under Self. He has said before that he doesn't want to coach forever, and I think he has started grooming Jeremy Case as his eventual replacement. Case is at a perfect age for that (37 I think) and has already started getting more responsibilities on the staff.
Unless something crazy happens, Self will leave the program in good shape. KU has a plan for the NIL, so the program should not falter in that respect. There's a ready replacement in the wings, and if Case isn't the guy, KU can always draw nationally because of the history and backing of the program.