Dreth is pretty decent. I refrained from using him until green tier. But I do remember the story of how I got Gunnar to where he is.
I got Gunnar at level 43 two seasons ago (Winterjól) and I could not level him up to green. You can get green divines at level 49, but I think you need two green legendaries? I’m level 76 and am just about able to breed platinum dragons. Generally, divines start getting a bit better at green, because you have 3 of the 4 spells.
I absolutely hated how weak my divines were when I was below level 49, so my advice would be to get to level 49 as quick as you can and make sure you’re in green tier as quickly as possible. For now, you can build up XP on Dreth by using the daily XP multipliers so that you will be able to get his maximum green tier level in one round.