Recommendations for Sign Up sites...

Depending on the target audience, you can use Product Hunt Ship https://www.producthunt.com/ship

if you attach a mailchimp signup form to any page, it's free up to 2,000 subscribers. most of the time i use a tiny DO droplet and a free MC list, and boom - a couple minutes and i've got a landing page to test my idea.

depends also on the software, but i've found more people are willing to sign up if they get something (like reserving their username) than just "signup to know when we launch" (which is boring and not as valuable to people).

https://upscri.be/