reverb is a bit of a funny example, since you won't need lowest latency for that (since it basically, simply put, consists of multiple delays)
i guess you want a reverb effect added to your dry mic signal and mix those two, right? or should bela only send out the "reverbed" signal?
you could also use something like puredata on bela, maybe easier to start with.
if you want to dive into the c++ side of programming bela, there is a knowledge base here:
https://learn.bela.io/tutorials/c-plus-plus-for-real-time-audio-programming/course-introduction/