I'm not a mech mod person, I'd much rather have control of my voltage. Partially because I'm not perfect with my coil building, and sometimes I make a sweeeeeeet 2 ohm coil, and sometimes I make an amaaaaazing 1.5 ohm coil...and with a mech, one of those coils would be fantastic and the other would be kinda meh. And those work well for me because I don't care for micro-coils or sub-ohming. I'm happy with my 2ish ohm coil and my ProVari.
As for RBAs, the smallest tank that I'm aware of is the GG Penelope. They're a bit spendy, and there is a learning curve to building them, depending on which version of the device you get your hands on...if you can get your hands on one. They're limited run, so they're fairly hard to find.
If you don't mind a bit bigger tank, I would definitely recommend a Kayfun/Russian style RBA. They're a bit bigger and a bit heavier, but they're easy to build and definitely deliver on the vapor and flavor.