I love this concept of proof-of-work captchas, but there's a growing number of tools and ways to bypass IP blocks via IP rotation[1], specially after the explosion of IaaS providers. How do you intend to tackle this?
There are free and paid list of all ip addresses from datacenters like https://udger.com/resources/datacenter-list, they probably existing for specifically preventing this, so maybe thats an option here.
[1] Some examples: https://rhinosecuritylabs.com/aws/bypassing-ip-based-blockin... https://oxylabs.io/products/real-time-crawler https://github.com/alex-miller-0/Tor_Crawler https://www.scrapinghub.com/crawlera/