250 W is too weak to go without gears.

Segway sharing bicycles have interesting solution, automatic mechanical transmission:
https://b2b.segway.com/e-bike/
No idea how it works.

Pinion MGU also exists. 9 and 12 speed variants with an electric motor. I believe ZF is working on one as well.

Definitely on the lookout for one to use as my commuter bike in the next year or so for sure!