This is working for me too, and overall, I trust Cloudflare quite a lot for these captcha and other firewall concerns.
I turned off the custom field question and activated turnstile and so far no spammy registrations (more than 12 hours).
However, this configuration was not straightforward. The hook file is not 100% ready. You need to add the code in a specific location, otherwise, it will give you a page error. Here's how to get it done easily.
I hope WHMCS team takes this seriously and includes Cloudflare turnstile in their roadmap very soon, instead of waiting for votes on feature requests.