While obviously a big pain in the butt, aftermarket EV computers are coming onto the market which could help solve the long tail of used EV software related nuisances.

Lots of learning from the EV conversion market will go directly into that ecosystem too I feel. But that said, I'm pretty sure that it's already illegal in many countries to tamper with the ECU. It's currently because you can change the emissions profile of the car, but I'm sure they'll just swap to saying it's about safety instead for EVs.