Knowing when to switch your main class?

[RyuujinZERO]

The main question is are you having fun with your class.

Maining another class you play better mechanically, playing it for hundreds of hours purely for the sake of content progression IMO is unwise if you don't enjoy that class as much as you did your old one. It'd be better to put the time into master the class you enjoy, than picking up a new class you don't. With that said unless you try out the other classes you won't know what you like the most, and I have found rolling other classes to be highly educational, showing me new tricks and ways of thinking about encounters that can be used to play a better game in other class roles.

But if you really enjoy BLM, you just don't feel you're doing a good job of it, then it's probably best to worry less about immediate progress and focus more on mastering it.

But if you still want to try out other class, which to try depends on what you enjoy and what you want out of it. If you enjoy the lightning proc mechanic on BLM and ranged combat, then BRD's main attack priority has a similar set of mechanics. SMN shares gear with BLM, though IMO, SMN combat is a little dry, basically amounting to stacking/spreading DoTs, and dropping fester. All 3 of the melee DPS are pretty much equals for output these days - MNK is fast paced and demanding, but rewarding with it's adaptive skill chains. NIN I can't speak for, having not levelled it myself, but superficially shares some mechanical similarities with DRG. DRG is a more classical chain combatant that emphasises off-GCDs to double-up their strikes, hitting predictably but powerfully and surprisingly quickly.