Thanks. Still waiting for the 46T→39T operation.

39T and 28T is the lowest gearing from Gates for an Enviolo hub. Considering the current 46T+28T worked for the last few months, it's not impossible 39T+28T might work… for a while as the kids grow heavier.

If the 39T chainring doesn't work well (good to ride uphill, but not on the flat), and she doesn't like giving up on the belt (and since a 12s cassette only provides ~330%, less than the 380% of the Enviolo Heavy Duty*), the only option would then be to move, or get a naked trike and power it with a BBSHD.

Besides Refully, there's also PréciAlps, or using an adapter. Provided it even works with a Brose mid-drive, I know the chainline could be an issue and the lack of a derailleur isn't for everyone.

https://lunacycle.com/luna-chainring...-bafang-bbs02/
https://ozo-electric.com/en/accessor...s01-bbs02.html

* or, provided the frame can take a derailleur hanger, use Sturmey-Archer's CS-RK3 to combine its three-speed gear hub with a 8/9s cassette :-) Could the hole where the missing snubber was be used to install a rear derailleur? "I built a custom 21-speed (3x7) wheel with this hub to suit my 1600W 58.8V BBSHD mid-motor ebike and it’s great! 500 miles so far and not missed a beat. Just remember to shift when not pedalling." (source)