Just accept it if they don't verify it anyway.

Make sure to have good KPIs not using the pure number of accounts. I think its absolutly valid to have it like that.

IF you really don't want to use a captcha, why not write something very basic specific to your page?

Those are probably spam bots filling out a basic form.

If you are OK with running a Google service, how about Recaptcha V3? No captcha to fill in, just validate the score it generates on the backend.

Is that the same one that sometimes asks you to click on motor cycles and zebra stripes in a loop and you never come out? (this happens often especially if you're on a somewhat uncommon IP address, like an IPv6 /48 with just a few customers on it)

That’s V2. V3 is always invisible and only returns a score. It’s not unusual to redirect low scorers to a v2 challenge, but afaict you have to set that up by hand.