Too heavy? You just run the motor in reverse which turns it into a generator and adds friction to do generative braking. There really isn't any added weight.

You can find YouTube videos of people experimenting with flywheel-based regenerative braking. They're completely impractical but pretty funny to see.

Motors are generators when run inversely:

Motors = put in power to get rotational movement

Generators = put in rotational movement to get power

You already have the heaviest parts on the ebike - motor and battery, just need some capacitors and charging circuit board which are light and not too big.

Cheap electric bikes I've ridden with regen breaking slow you down quite a bit.

It's like 3-15% energy reclaimed, not efficient

If you need a minimum speed to maintain balance, how are you going to pedal while stationary?

Better to have active regeneration, where you pedal a bit harder on flats to top up the battery for hills.

One needs a certain amount of forward speed to maintain balance easily

Not true. One just needs to practice static balance. It's a great skill to have so everyone should learn how to do it.

Also you can use the lowest gear going almost a walking pace and climb up really steep hills. That's how we do it on MTBs. It also requires practice and good balance.