I did something unpopular and just didn't have a captcha, I just read up on creepjs etc and rolled out my own which is just browser state analysis, basic ip check (abuse lists only) and PoW. Haven't had an issue with a single bot registration (yet).
There's hardly anything you can do to stop someone determined enough to spend money to spam your specific website. These kinds of captchas do raise the bar somewhat, but every single one of them is ultimately bypassed by paying people to solve them for you.
bots get pruned after an hour since 100% of the bots fall into the same trap, giving it a delay makes A/B testing really difficult and breaks most AI strategies.