Discourse’s official hCaptcha plugin can be a huge help here. It’s specifically intended to help mitigate bot signups.
(I would personally also love to see Discourse support for Cloudflare Turnstile, given that Turnstile’s free tier includes a frictionless non-interactive mode, whereas similar functionality with hCaptcha requires transitioning to their “pro” pricing tier at $99/month—which is absolutely pants-on-head ludicrous for self-hosters.)