Mirotic is a shooter, so he fits with Pau just fine. While Gasol might be able to spot up for three, you don't want him spacing the floor for Lee. It'd be like Kawhi spacing for Parker. And LMA was plenty fine for a defensive center, especially against bench units.
As far as my preferred units, I want the Spurs to play small with their bench with Lee as the fifth big. I really like the Anderson/Bertans forward combo, and if Garino makes the team, he'd be another option along with Simmons to rotate in there. I want a return of four out, one in like with Neal, Manu, back-up three, Bonner and Splitter units.