Some more software work is done - I think I have everything implemented now. Motors seem to behave well - I mostly copied the throttle/brake handling from the hoverboard firmware. Tomorrow, I'll take it for a test ride!
Also - good thing I put both double software serial as well as single hardware serial with a 2:1 multiplexer on my board. Turns out software serial can't reliably handle Rx with 115200 baud, so I switched to hardware serial now, at the cost of my debug output :D