The maximum pots one can water with an iridium can in one click (and still be able to collect) is 16, I think. So it's probably doable to have 32 coffee bushes indoors and water them in 2 clicks. But yeah, I generally aim at the retaining soil.10 coffee plants isn't enough for constant coffee buff, is it? I tried that once and I ended up with like 20+ plants. (I also hated the manual watering lol, although deluxe retaining soil might have helped if I was doing it on 1.5.)
I personally prefer +2 speed to the defense buff, since I can dodge enemies easier and not get hit at all, but ymmv.
I don't mind watering that much, I like to have indoors plants anyway (a small herb garden in the kitchen, some tomatoes, aubergine etc.) but collecting all that coffee would be a nuisance, I think.
I don't generally use speed buffs all the time, only in very limited circumstances. I have dyspraxia (which simply means I'm extremely clumsy, drop things and suck at hand-eye coordination) and managing a fast farmer is even harder than a slow one. I still haven't caught any of the legendary fish, and I die more often when fast than when slow and armored because I can't react quick enough.