Everything gets more competitive in D.III and D.II. If you want good competition night in and night out, promote. If you cannot promote, you will unfortunately have to deal with bots and people rebuilding in the way that you described. There are definitely ways to train and rebuild with a roster of 8-9 per game, but until training minutes/game shape implications are altered in some way, it simply doesn't make sense to send out 12 guys if you are trying to train and develop players. When you think about the USA leagues look at it this way:
NBBA = NBA
D.II = G-league
D.III = College
D.IV = High-school
Obviously when you get to D.III and above the games are generally more competitive and people take it more seriously. In D.IV it truly is like lower tier high school basketball and it is a bit of a mess. Promote and this problem is mostly resolved.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs