I don't see the point to use it's own dns server for a single VPS. It is recommended to run at least two authoritative dns server for a a zone and it seems really strange to use a dns server which just references it's own IP address.

If you just want to learn how to configure dns records I would recommend something like https://desec.io/ . They provide the dns server infrastructure and you can configure the zones as you desire.

And from there to the linux pc? Why not just add tunnel to your external device?

What do you mean "a router that is using a wireguard vpn" ? Why not just add your device as an endpoint in wireguard?