Lvl 5 trainer is great already for your division, maybe even too expensive unless you don't need to compete and just want to train. As long as you can train him optimally and don't have 3 better trainees, you can keep training him until you have better trainees. Would be a shame to pay your trainer while you don't have trainees.
And I agree that for lower divisions a Superstar potential is as high as you should go for, aim for 50 TSP for 18yo (at start of season) or 62 TSP for a 19yo, you should be able to get one for under 100k. You can train a superstar SF to 130 TSP so you really don't need a higher potential than superstar (also you wouldn't be able to afford the salary a capped MVP or higher player in DIV, those are 100k+ salary).