Harris will be 27 in a couple months. I'm not sure his game has any room for improvement at this point, he is what he is. I don't think he is worth a max deal.
Simmons and Embid both have room to grow. Embid is much further along, and as long as he stays healthy will be a dominant force going forward. Simmons is another story. I am really disappointed in the lack of growth over the last year. The same issues that the Celtics bottled him up with last year, the Raptors are doing this year.
He is a terrible shooter, and now appears to be scared to shoot, or scared to be aggressive in going hard to the basket because he is scared to expose his poor foul shooting. What is the point of being 6' 10" if you are not willing to do the dirty work. I'm not going to say he has poor work ethic because I honestly don't know if he is putting the necessary time to work on his shot or not.
If you commit to Butler and Harris, you are in win now mode and you can't wait for Simmons to develop a shot, so you almost have to trade him. Sure he is a great open court player, but the playoffs is half court basketball, and Simmons is a huge liability, and it may be years until he can shoot it well enough that he won't be a playoff liability. Butler and Harris might already be too old by then.