My advice to you is before you do the split, you need to sit down with your team and logically plan out on paper the end state of your Orgs, how they should look when you reach your ideal state. Naming conventions, tags, auto VPN considerations, what goes where, & who has access to what. Also, audit trails. When you do a split all of the change logs up until the split remain in the parent org and the day of the split the change logs start in the new org. I would also do an admin and access audit BEFORE the split(s) or else you will be doing audits for access on each org after the split, loads of fun. Pay particular attention to any camera and sensor admins as these don't get moved in a split.