I also run a 980ti. Pretty nice rig. Quite old too, tho. Do you display Valheim in 1080p? since i use a 4k desktop and monitor, i cannot go for 1080 ; it just looks too blurry, so 2K it is (2560x1440), but then i need to remove most feats for it to be smooth enough. Still looks better than 1080 ;)

Anyways, if it can help, i get mostly 40-60 fps, and snowstorms seems to be the worst case. Most often i get 55-60 fps, not exactly stable, but stable enough. Ofc in the dense base with all the stuff around it gets on the lower side, around 50 at worst, but nowhere near 30 fps. Not using any of the "gfx-commands" stuff you run. Just fullscreen 2k, with an off-screen on 1080p running twich or VLC or anything else. For a 6 year old rig, can't complain.

Vulkan seems better.
Drivers are probably a year old. Yup. No more updates for that model anyways.
Proc is a i7-4790K CPU @ 4.00GHz. Sounds like a hair dryer when running Valheim. You are lucky to have watercooling ;)