While I don’t think anyone should be allowed to take a 2150Wh home made battery on a train, I’m not sure why battery capacity should affect classification. Limiting power delivery makes much more sense.

Quite common for Bosch powered cargo bikes to allow fitting two 800Wh batteries and you can always carry spares. They’re just horribly expensive.

I’m very glad my US spec e-bike is 28mph/45kmph rather than 15mph/25kmph limited as I feel much more comfortable taking the lane closer to car speeds on residential roads even if I rarely go above 20mph.