You may not even need to buy a domain. You could use one like me.internal with something like split DNS - where if you come from your internal network you get one response (internal 192.x.y.z), and when you come from tailnet you get the tailnet addresses. You would need to set your DNS server to this one on your home network and the tailnet, ensure it’s set up to forward everything it doesn’t know you your ISP resolvers or like 1.1.1.1 as you normally have set up. If it’s all private and you don’t want public access then a public domain doesn’t benefit you.
You may not even need to buy a domain. You could use one like me.internal with something like split DNS - where if you come from your internal network you get one response (internal 192.x.y.z), and when you come from tailnet you get the tailnet addresses. You would need to set your DNS server to this one on your home network and the tailnet, ensure it’s set up to forward everything it doesn’t know you your ISP resolvers or like 1.1.1.1 as you normally have set up. If it’s all private and you don’t want public access then a public domain doesn’t benefit you.