The game is probably as optimized as it's going to get. At this stage to eke significant performance gains, it seems like they'd have to rewrite the renderer to parallelize more of the drawcalls. Since this is still technically a DX11 game (Vulkan API is only a compatibility option and garners no real performance benefits) there's still a huge emphasis on single core load with both Driver overhead and the primary render pipeline stuck on one pipeline.

Converting to true Vulkan multicore/multithread optimization is an engine-level change, likely far outside the scope and perhaps experience of the team, to say nothing of potential legal ramifications when changing the Engine in such a way (I don't know the EULA of Unity, I have no idea how much you can modify)

Given the nature of Valheim, it's one of the few games that could really benefit from AI driven upscaling (FSR or DLSS) but even that isn't a quick fix.