How to handle multi-tenant SaaS with custom vanity domains?

Hey folks, anyone working here on a multi-tenant SaaS product?

I am currently building a web page builder SaaS with Nuxt.js that allows user to manage their to sites via app.mydomain.com and host their site via a custom subdomain such as coolCustomer.mydomain.com

Additionally, I want to allow them to point their own subdomain to subdomain hosted on my application i.e FROM coolCustomer.mydomain.com => docs.coolCustomerDomain.com

I have done quite some research and read a lot about reverse proxies (nginx, caddy, something with cloudflare dns) and quite cumbersome ssl certificate creation process.

Now I have read here (Introducing Wildcard Domains – Vercel) that Vercel does exactly support this but I am not really sure.

What would be the easiest solution here?

P.S I know that this issue might be more server related but maybe someone here has some experience