Any further thoughts on this?
I started my pre-reqs 2 years ago and I start dental school at the end of this month. But I got a degree in music several years ago, and I plan hold on to it through my life. And among the reasons I chose to return to school for dentistry over medicine: I figured a dentist is slightly more likely to be able to have a career, family, and hobby... whereas watching my father, medicine seems like a career + choose one side.
One thought I don't think has been mentioned: if you take the gap years *after* school, your loans will be accruing interest for those years. On $300k, that's maybe an extra 30-40 thousand dollars lost, as a baseline--and once it's capitalized that's even more money lost down the road. And your skills will atrophy. And although a dental chain may not care, a potential sole practitioner hiring you as an associate may wonder why you took time off, and a bank who might give you a business loan may wonder about the gap.
But in the grand scheme of things, since you already know what you want, I don't see why you should delay at all, before or after. Getting your career up and running ASAP will be the best thing for finding balance for English and music. Either 1) you assume that dentistry will give you time for outside interests in which case delaying makes no sense... or 2) you assume that dentistry will not give you ample time for outside interests in which case delving deeper into those interests now only to give them up in a year or two makes no sense either. Sure, you'll have less time in dental *school* itself. But that's going to be the same game whichever path you choose.
I say, don't sell yourself short. Make a decision to have time for English and music in addition to your dental career, and get them all going soon. (And anecdotally, I can say that I became a better musician once my life stopped focusing 100% on it.)
That's not to say that gap years are bad---I've written several things on this board in support of them. But I support them for confused or unmotivated people, not for people who already know what they want.