I am trying this since last 2 days.

I build an example app using a very detailed tutorial here - https://john2x.com/blog/wildcard-certs-from-lets-encrypt-cert-manager-ingress-nginx-gke.html

What I have working right now:

I have an app running at demo-app.example.com wherein users can have their own subdomain like john.demo-app.example.com I got the wildcard certificates working as well

I have two questions now:

1 - How do I allow users to add their custom domain pointing to the subdomain in my app. When I just create a CNAME pointing to a subdomain in my app, it does not work, I think because of how the whole thing is configured.

2 - I want to move from demo-app.example.com to just example.com to the top level, so how should the DNS zone be configured for this?

Finally, given the requirements, is Kubernetes the best I could use or I would be better off using something else. It is a NodeJS app btw.