I'd have thought a little onboard dimmer module (motor rated of course!) would've been the ideal way, if you can get it to fit.
Some lab beaker stirrers we used to repair had dimmer-style speed controls (universal motor) and ISTR they were fussy about the triac contained, DV/DT triggering and all that..but a purpose dimmer, somtimes sold as a 'drill speed controller' will be suitably designed. PS..some dimmer controls might make the motor buzz annoyingly, though a small motor should be ok IMHO
https://www.ukelectricalsupplies.com...ive-dimmer.htm