Martial Arts advice

Mea culpa. I should have expressed that this was based on my outsider's preconception of martial arts as a whole... compared to something like parkour or breakdancing, martial arts im geral seem rigid and unquestionable; if you don't do exactly what Sensei says, you're not doing it right, whereas Capoeira has absolutely none of this, on the contrary; if you're not expressing yourself and experimenting, then you're not doing it right.

Well, now that I think is spot on. I don't want to fall back on lazy stereotypes but there absolutely is a marked cultural difference between the traditional Japanese arts I started in and the Brazilian atmosphere I train in now.
Even in the BJJ gym though there is definitely an emphasis placed (rightly IMHO) on adhering strictly to doctrine in the initial phases of your training career. It's only as you progress that you will begin to open up your game and start subtly altering the basics to suit yourself. This was also true in the TMAs I studied but there was much more weight placed on the conformity aspects and, of course, the whole environment was much more formal.

Interestingly, more and more BJJ is working it's way into Capoeira games; training in Bahia last year opened my eyes to a level of grappling in the roda that I hadn't seen before; combined with a more offensively-positioned base that is almost certainly also borrowed from BJJ.

That is interesting. To the best of my knowledge there doesn't seem to be much if any cross-training going the other way but, AFAICT, there isn't a lot of overlap of curriculum in any case. Happy to be corrected on that point if I've got it wrong.