So I'm just about to get my relic on BLM and relic+1 right after that... But spending myth tomes is a big investment time-wise. So I want to be really sure about this decision.

If I want to progress further on to Coil, the needs become more and more specific, and how does BLM do in this department ? Will I be needed, or will I have to fight for one spot that might be open ?

I have heard a lot about Summoner, Bards and Monks having the best dps, and that Bards and summoners are a better pick over BLM cause they not only do more dps (and can do it easier), they also have much better utility abilities...

I have a set of full DL if I change to Summoner, so I only would have to get a weapon on it. Should I stay on BLM or SMN ?

You can answer "choose the one you like", but I want to know the harsh truth too