So long as they aren't eligible until a new season starts (which I don't see as the hugest deal, but coaches are beside themselves about the idea of players coming in midseason, which is an easy and reasonable enough limitation to create) does that really matter?
I tend to think managing the line of who is "too pro" to be playing in college is a problem that would take care of itself. Players with NBA opportunities are going to take them, even if it's for less money, and "real" NBA players are going to be under contracts they aren't allowed to break.
Just do 5 years to play 4, with the clock starting the year of first college enrollment or the season starting after the player turns 19 (to keep way too old Euros out), with any season in which the player was paid to play pro ball (either in the US or abroad) counting against years of eligibility.
James Nnaji would be a senior at Baylor, rather than a freshman, and thus a much bigger gamble and a much less valuable asset, reducing the incentive for guys like that to flood back into college.
Players routinely changing college teams every season and having no roster continuity from year to year is a WAY bigger problem. I hope NIL contract buyouts can make progress on that part of it.