I'm no lawyer, but couldn't a change to the letter of intent an athlete signs take care of any legal challenges by requiring the athlete to remain as a part of the team until the season including bowl games over. It used to be the standard that transferring would require sitting out for a period of time.That makes sense but would also immediately trigger a lawsuit. The NCAA basically created a wide open free agency system and there is no way to put the toothpaste back in the tube now.
Changing the portal dates isn’t going to help because guys who have already made up their mind to transfer at the end of the regular season aren’t going to stick around for the bowl game even if you make them wait a few more weeks to enter the portal.
And forcing them to sit out part of the following season with their new team isn’t going to hold up to legal challenges.