How do you navigate the sign up journey?

The hardest thing to get right was getting people successfully through sign-up.

Initially I started with one time links, had a few successful sign ups and inbetween them would be a couple 'unauthenticated' emails. I would reach out via email to try find out why they didn't complete the process, but get no response. I eventually found out that some email providers burn the one time link when they scan the email.

So I switched to an emailed OTP code. That fixed the broken links, and the conversion rate improved, but there is still a drop off at this trigger point and I can't get why.

Is this common? My sign up process is already straightforward as I dont ask the user for any information that I dont need, not even their name (it's optional only for a more personal in app experience), and the encryption key can only be generated for them after they have verified their account with an OTP or link.

Have you found anything that helps people come back after the email step?