In December of 2021 I hit SFMM and KBF in one trip; here are some of my observations/suggestions:
-I flew in and out of Burbank International Airport (BUR) for substantially less money than flying into or out of LAX, via Southwest. Yes, flight times are limited by comparison, but what they had available worked for me. Rental car prices were also substantially cheaper at that airport.
-With Flash Pass Platinum, I would imagine you will only need one day at SFMM. I agree with two days at KBF though, especially if you plan on taking in all of the non-ride things to do there.
-Don't forget to check out the Santa Monica Pier and Pacific Park while you're in the area. Only one credit, a so-bad-it's-good Morgan micro-hyper (full scale track and trains but a tiny kiddie coaster layout), but the park has historic value and also plenty of nice dining options in the area.
-If you're a credit whore, there's also a few credits to be had at nearby Castle Park, John's Incredible Pizza, Live Oak Canyon Farm, and Apline Slide at Magic Mountain, the latter of which has an honest-to-satan real Apline Slide along with their mountain coaster.
-Since you're only doing the LA-area parks (and ignoring the true SoCal parks down near San Diego), I'd honestly get one hotel that is centrally located for the entire time - this will give you a little more flexibility without having to work your schedule around check-in/out times. When I did the trip, I stayed at a mom-n-pop hotel that was in walking distance to the beach and dining/nightlife options.
-Double whatever your expected fuel budget is.