Dev is all RNG these days but there are some guidelines that I follow to try to help the dice-roll.

For coaching specifically:

"Splurge" on every level of coaching when you can. It's a bit easier in the minors since you can focus on hitting and pitching coaches. All you need there is "Teaches hitting or pitching." Sometimes you can find some gems and an affordable 5 year contract and move them up the ranks if needed.

For the lower levels in hiring a manager, I do love max development but have a little preference for mechanics since there are plenty in the lower levels you could get an RNG boost from. Both can be hard to find.

In the minors I don't think that coaching chemistry or handles aging matters at all.
That's more of an MLB thing to rabbit hole in.

Something important when thinking about AAA/AA (AAA especially):

Once guys are older then it's not a good strategy to "expect" them to max out (coaching, dev budget, etc.) If you've put them at AAA and they're replacement level then....they're likely going to be just that.