Best membership solution for 20,000+ members?

Hi all,

Currently have a client with over 20,000 members in their existing site (it's a Weebly website) and they want to migrate to Webflow.

However, most of these are free users. It's just simple gated content.

The problem is, the apps I've found (Memberstack, Outseta) all have pricing based on number of members, and the top-tier plans are $100+ /mo which exceeds my clients monthly budget (they're just a single author).

Does anyone know of any membership solutions that offer unlimited members for a flat fee? Just for rendering simple gated content.

Thanks!