I feel like I remember hearing about a server side mod that would share map progress with anyone that joined. The internet probably knows more about it than I do.

As for a simple solution, build a cartography table?

OK re-read your post... You don't have to rent a server, you can host your own for pretty cheap. Pretty sure a raspberry pi can host a Valheim server. Or any old PC you have laying around. Worth a shot.

I pay $35 for 3 months on g-portal. Not really very expensive.