I don't think I would put a 7yo in MP3 unless his phonics/reading, writing stamina AND comprehension were far above grade level. Plus, with your older son needing a little extra time to work out comprehension despite being hyperlexile, it seems even less like the right fit to have them both there. My eldest is quite gifted, and I have kept her right on grade level in MP2. I've been able to witness firsthand how time and maturity, two things over which I have no control, make all the difference between frustration and ease. Now, with that said, a lot can happen in 7 months for your 2nd child. Even with math being a year ahead, you get into issues with Latin and science and Classical studies later that really require a level of maturity not present in a young for grade AND full year younger student.

Cheryl Swope has a great Sodalitas 2018 video on working at your children's independent level. It was so convicting. I'm curious if she would have anything to add about a Simply Classical approach for both of them. My youngest is finishing SC B, so I don't have anything meaningful to add about SC2 or 3.

For the K-er, definitely do 1st Enrichment over next fall. My 5yo needs books at his level or up to 2 levels above for listening, but that doesn't mean grade levels.