you can map anything with a virtual folder path over the original save location path. make a clean backup before you do anything else, then take a look at https://www.computerhope.com/mklink.htm or do a web search.

There may also be configuration or other ways to do it, this is the brute force answer.
For examples of how it works, the idjits that made my PC put windows on the SSD and I moved all the constant read/write junk folders to a rotating drive with this tool.

another way is to make a little batch file that runs valheim, and when it closes, have it copy the saves to another place. This gives you backups as well as automation.