Interesting project you have. I am also in Washington State, have a commercial Italian motor scooter using four 12v 38ah SLA (actually AGM) batteries, with an 1,850 watt brushless motor. Interestingly, three of the four batteries are originals from 2001; actually none ever failed as the fourth was removed to replace a battery in another scooter. I desulphate them regularly and attribute their longevity in part, to that. The German speed controller does have regenerative circuitry.
That sounds like a neat scooter! I went through a scooter stage. My nieces, nephews and grandchildren would all come over to ride them with me. They were all cheap kid type scooters. We had a couple gas scooters and a couple of electric scooters. I also put an 80cc 2 stroke motor on a mountain bike. That was fun! Fortunately I sold it before I got a ticket. I sold all but one of the scooters, and one motorcycle.
We have a small airplane; the batteries for that cost a small fortune these days. Because airplane batteries sit for long periods between charges they often last only a couple years because of sulphating. The desulphting charger has really paid for itself many times over.