I use B4A-Bridge and wifi, simply because I run on many different devices and it makes it simpler to switch.
As for the devices, it's my current phone and tablet, then my old phone and tablet, as well as a pile of old phones from family and friends. Seriously, if people around you know that you are interested in old Android devices, the devices will flow to you in a steady stream as your friends upgrade and wants to get rid of old stuff. No need to be super rich.
I switch between debuggers. Mostly, I use the fast debugger, but, as others have said, the ability to stop on the line where an error occurs makes legacy indispensable. Also, occasionally, changes made to the code does not seem to take effect, and switching between the debuggers, building in both, takes care of this. If fixes were made to address those issues, legacy could be dropped as far as I'm concerned, but, until then, I prefer it very much if it remains.