Amazon does have 3.0.0.0/8. Granted, it is noted as two separate /9 blocks, but it's all Amazon.

Perhaps a better use of your time (because de-balkanization of the internet would be more of an academic exercise than a mass market reality; consumers have already self-selected into social media and crappy LLM outputs which is a far bigger matter and driver than ephemeral resources or reverse DNS) would be making your TLS work in a way that doesn't allow for easy abuse. So automatic redirects, HSTS, and a certificate that is standards-compliant.