lol, using AVX2 hash grinding is not difficult to set up, and if you want to go to a little more effort you can turn an old S9 into a hash grinder unfortunately i think that once there's money in it, your little antispam mechanism is toast

Not sure how long it will take tbh, perhaps several days

I looked into this, bitcoin miners are not designed to calculate a single sha256 sum so I don't think they can be adapted for nostr POW. And yes, but you just need a difficulty adjustment and all is good.