Olympus alone at lv 8 seems to be most critical. That can give very good hotspots by itself. Next best ones I’ve seen are Floating and Castle Mountains. Floating Mountains makes a great saltpetre farm, too.
Lot of luck involved though, yep, on size of biomes and placement. Finding it good to not take TOO many high elevation biomes on these. They take up a lot of space, and some won’t run together to create big spots. Some normally lower elevations ones though can get raised up enough though if merging with high ones to be high enough for diamonds.
Let’s try another experiment, I’ll post how this works out - T6, no rivers, standard caves on all.
Tier Mountains (never tried this one, so this is an experiment)
lv 6- (This is tricky, not many good high ones here, so I tend to pick to get other stuff or at least stuff that won’t mess up the high ones too much)-
Gleam Spiral (give myself a little hunting area with the experiment)