OP wanted "instant self signup". That doesn't work when malicious actors are trying to register accounts with stolen credentials. The verification flow is required because of the amount of pressure from malicious actors against both free and newly-created accounts.

"Give access now, cancel if validation fails" doesn't work either - so long as attackers can extract more than 0 value in that duration they'll flood you with bad accounts.

Well, then give me self-signup followed by a clearly outlined verification flow that I can follow. If you show me a form where I can upload my passport or enter a random number from a charge on my card, that counts as "instant" enough. If you really need to make me wait a couple of hours while somebody manually reviews my info, fine, just tell me upfront so I can do something else in the meantime. It's all about managing expectations.

Besides, Amazon hands out reasonable quotas to newly created accounts without much hassle, and they seem to be doing okay. It's not about abuse.