I agree with the idea that reading from one subject area (or series) at a time is probably more efficient/better, in terms of building up vocab at least. However I don’t have the self control for that hahahaha.
I think there are some perks also to reading a variety of things at a time as well though. I’ve found quite often over the last few weeks that I will look up some vocab in one thing I’m reading, only to have it pop up again in something else. I feel like for some reason seeing it in a variety of different contexts helps me to remember things more than say, coming across it a couple of times in the same manga.
However, I’m guessing this probably only happens so much because a) my vocab is tiny so I can still come across everyday vocab with a lot of crossover that I didn’t yet know, and b) even though I’m reading a variety of things at least most of them have a fairly heavy ‘slice of life’ focus. I’d imagine for example that if you already had most ‘everyday’ vocab covered and decided to read a dense SF novel and say, some ye olde fantasy…then likely there would be limited crossover there.
TLDR: reading from at least the same genre is probably better but if your vocab isnt massive yet and you’re not reading super specialised things then it’s probably not worth worrying too much about. Read what you enjoy! 