Help and advice to set up a custom domain system for our users.

Job ID: 32218691

Budget: $30 – $250 USD

I'm a sole frontend developer working on a "site builder" like application. ​
At the moment, users can create "sites" and host them on a subdomain "*.example.com" (All subdomains are stored on a Postgres database with a Graphql backend).

The app is hosted on Vercel and is linked to a wildcard domain.
The subdomain is then parsed from the host on the frontend and used for requests.

Now I'd like users to add their own purchased domains to my site, but I have no idea how to go about this. So I need someone to help me set this up, and ensure that the domains are secured with SSL.

I also realize that Vercel most likely can't support my use case, so I'd also need guidance in finding a tech stack that fits my needs.