OK, thats a "Whao!" Looks like it's right at the edge of the Ashlands, and is likely to be a lot of Plains zones around it. Not somewhere you want to go with Swamp tier gear.

You might try focusing on thecrypts in the swamps North and East of where you found the vegisr that gave you that location. Every map has multiple boss summoning spots all around the world but the vegisrs will show you the one closest to you.

On my wolrd I had a Bonemass almost as far South as yours but when I was farming Iron in swamps North of Spawn I found a vegisr pointing at another Bonemass that was actually closer to spawn than the other.

If you are comfortable with a bit of cheating you might consider putting your worlds seed into the map generator at https://valheim-map.world/ to see if there is a Bonemass closer in a diferent directing. The map generator can be set to hide everything but boss spawns so it would limit the spoilers, but it would at least let you know if there was a closer Bonemass in a different directing.

Good luck