Going Down a Steep Hill?

Would you not technically lose any benefits on the uphill that you would gain on the down ?

This is where mid drives win out every time :)
Thats a really good point. All of the weight you would need for the big DD hub motor to overcome its lack of torque, and its inability to use the gears ... thats a bill that has to be paid on the way up the hill.

To reiterate what @HumanPerson said, regen is typically just a small return... and it ruins your ability to coast because the bike is either on-throttle or its engine-braking. The only way it can recover meaningful energy is a) You ride where there are very long descents that you can use the engine braking and no brakes to recover energy for an extended period and b) you spend the time and money to buy a premium motor and controller solution that lets you dial up regen to be very strong - way stronger than is normally used. Strong enough that you can roll down a long steep hill and never need your brakes.

You would recover your energy all right... but your riding experience would suck. It'd be like one of those vibrating hotel mattresses that you feed coins into. Another bill that has to be paid: Regen so strong it actually does a lot of work is hard on the frame dropouts (and the frame).

Everyone loves projects, so if you decide this one is yours then have at it. But definitely not something I would advise putting effort into for any practical benefit.
Mate, regen is way over hyped on the ebikes...seriously.

You'll only ever get around 7% back on a good day.

If people arte good with that, and some are good with that small amount and some people are not good with that so they turn regen off
on some brands of ebike motors...make sense?

You're better off just using a second battery or getting a huge single battery. . . .

That's not just my opinion...there are alot of builders/ebikers that will say the same then there are others that will just shoot air out
of their blow holes lol

Do what's best for you ;)
Regen as a battery-extender seems to add small beans, <10%. But I would like that if it also gave significant rotational retardation. Academic to learn because I am satisfied with dealing with downhills with brakes and pre-planning.