Typically you want the client to own the domain. However I personally like owning the server. An easy way to bridge these is to host your production servers for your clients on your own domain and server using a subdomain, and then provide CNAME DNS instructions for your client. Then, their domain will point to your subdomain which points to your server IP address.

Another tip here, if you are building a lot of micro websites (sites that require few server resources) it is likely going to be the best choice to run a reverse proxy on your server. This way you can have many domains and subdomains all pointing to the same IP address, and then provide CNAME to your clients if you want to run them in production.

A note of caution, though: Make sure you have strong backups and this system, as I said would only support low system resource websites. Anything more sophisticated and I would highly suggest you run a dedicated server. Possibly that could be an "upsell" to add value.

Anyway, good luck. Full stack is pretty massive especially if you loop server admin into the stack. My parting advice from someone who is trying to be full stack solo dev is to ease into it. By that I mean there are many freelancing opportunities out there to work with a team: either a front end dev, back end dev, or sys admin stuff. It's likely going to be easier to pick up the skills by learning from others than to only rely on tutorials and google or whatever else you use. Cheers.