I don't have BRD or BLM, so can't comment on those specifically. I do have a well-geared DRG, SMN and MNK, however, and really they all feel about the same difficulty to maximize, with exception of feeling a bit more comfortable on DRG since I main that one and have had it at 50 the longest comparatively.

SMN has 3 main DoTs to maintain, filler with Ruin I/II, bursting with Fester. Little intricacies such as adding Miasma II on single target when you have Contagion up, fitting in 2 Festers under Raging Strikes, and using off-GCDs after instant casts to maximize uptime, etc.

DRG has 1 main damage buff, 1 resistance down debuff, 2 DoTs, and quite a few off-GCDs. The general rotation is about 24 moves long, but fairly easy to master. Only 2 positional requirements makes it a bit more forgiving than Monk. Superior burst AoE when used properly.

MNK is my latest lv50 so not so much insight into the intricacies of this job yet. I've found it to be pretty hectic compared to DRG, once you get GL3 and are cruising through the GCD you have to be pretty well aware of your positionals and status of buffs/debuffs, thinking 2-3 moves ahead always. You buff yourself with Twin Snakes and GL3, debuff the enemy and manage 2 DoTs (3 if using Fracture, this is pro, I'm not there yet :P).

If I had to rank them in difficulty, it would probably be MNK > SMN > DRG. But this is a bit biased, as I am very confident on DRG, and still learning a bit on MNK.

Good luck!