So by your own admission, 2nd round picks are not guaranteed any particular money like first round picks where there is a guaranteed two year contract (need for a good agent and bargaining is key for 2nd rounders and undrafted players). With the way NIL works in today's era, if you are not projected to be a guaranteed first rounder, it might be best to go back to college and take a higher guaranteed one-year NIL.
Of course there is the NBA rookie minimum for any 2nd rounder or undrafted rookie who is able to stick around in the league, but that minimum is much lower than some are likely to get in guaranteed NIL monies.