I'm not the know all person here, there are people that have played far more than i have. However, there are some basic pieces of advice:
1) In a typical season, you're not going to have a guy pop a ton. I heard somewhere it's 8-10 at age 18, and elsewhere 10-13. So if you have a power forward with respectable rebounds, inept inside shot, shot blocking, and inside defense.... How realistically will you make him a guy that is strong+ rated in all of those areas over the next few seasons? You're not likely to have value.
2) When in division 5, there is no point in getting veterans in positions that you plan to improve. There is also no point in training guys at all 5 positions to take over for veterans. You should take advantage of your chance to advance in a weaker division with prospects that you can continue to use as you get a better team.
3) When developing players, realize the rates of return. a star potential player at age 18 improving from 48 attributes by x amount is not going to return as much as the 18 year old with 55 attributes at the start improving to the same x amount. you might purchase for 1k and sell for 10k the first guy, and purchase for 10k and sell for 100k the 2nd guy.
I would advise you to cut your prospects or transfer them if you believe they have enough value to make it onto another team. I'd then go to work on finding guys on the transfer market, maybe 2-3 guys for now, who are clear improvements and slip through the cracks to you.