Without getting crazy technical, what controller would work with a 48v battery paired with a (measly) 350w motor? My Fiido has potential, but upgrades are so limited. Searching has come up empty,...

I think you could consider a 350W or 500W controller.

BMS Battery has a 500w controller, B09P that can be used with 36V or 48V, but the connectors for attaching it to the motor, throttle and display may be a challenge. Also, it doesn't indicate if it's "self-learning" which is another issue. Have you asked CS @ Fiido? If they can't help, and not to be the type who is asked a question and answers something else, but I would consider changing the motor (maybe display and controls too) if I were doing a modification. Endless Sphere (ebike build threads, first page) has a report of someone doing a similar "complete" modification on a Fiido (possibly YouTube too). DW probably has done this type of change. Hopefully he'll respond.

The motor I'd leave alone for now, but the battery is 48v, so a better controller, paired w/a different display and throttle would be the best option. I upgraded the chainring from 48t to 52t, but the freewheel is 34mm threaded, so it's a no-go for a smaller one. Eventually I'll probably go 58t, or even 60t on the chainring. I'll check out "Endless Sphere" this evening. Thanks!!! 👍