WARNING: Post is very long and technical.

No issues here; playing on a 1080Ti, driver version 456.71. Game is installed on a Samsung 970 EVO M.2 SSD. (Secondary drive)

The only few pieces of advice I can offer you and anyone else having issues with hardware like that are as follows:

1. You've already completed all the hardware and software (OC) related things I'd recommend first. Good job there. However, try popping up the nVidia Control Panel and see if Valheim has its own entry listed in the "Manage 3D Settings" section. If you don't see it listed there, add it and try running the game again.

I've seen the lack of a game in this list cause similar issues before, though not on 30-series cards and not with this game. If the game is listed there, and you've made any changes to it (or global changes), revert everything to default if you haven't tried already. This includes using that other nVidia-related program whose name I can't recall at the moment. (Though a "Clean Install" with the nVidia installer may wipe all profiles and settings anyway.)

This is important because these are driver-level changes that will superede anything you have set in-game, and can cause conflicts. (Such as forcing anti-aliasing in a game that doesn't support it.)

2. If you're using "GeForce Experience" or any other program to update your video drivers for you, stop letting software do this. It can and often will cause issues, although the issues it can cause are often BSODs. This also includes letting Windows Update install them for you.

3. You mentioned clean installs of the driver you're using.. when you say "clean install", did you simply run the installer again and allow it to do its thing? There's a very lengthy and annoying process to make sure your video driver installations are handled properly. Any tech forum anywhere will recommend you do this, so I'll list the process below as best as I can remember it.

Note that this does involve disconnecting from the internet and booting in to Safe Mode at least one time. Also, albeit optional, you can use "DriverSweeper" during this process, as it takes care of a few steps automatically (file remnant removal, registry entries, etc.).

1. Download a known stable version of your driver (and DriverSweeper if you're going to use it). Often the newest version isn't the best to use, especially in nVidia's case. They rely a lot on their community to flatten bugs and get a stable release out. Save this file somewhere easy to access, like your desktop.

2. Disconnect from the internet to prevent things from updating your driver for you, or turn all of that off. Now, start uninstalling anything nVidia, with the Display Driver last. Obviously this is going to cause your display to weird out and revert to the Windows default display adapter driver, which is fine. It may ask for a reboot first and then install the default driver, which is fine.

NOTE: If you happen to have an nVidia Chipset, do NOT uninstall this driver (the chipset drivers), or allow DriverSweeper or anything else to do so. You ONLY want to deal with your display driver.

3A. Reboot to Safe Mode (no networking). While in Safe Mode, if you're using DriverSweeper, run it now and allow it to remove anything it finds nVidia-related. Have it reboot to Safe Mode again once it's complete, or just have it remove everything and reboot to Safe Mode yourself after checking one more thing. It may force a reboot, I forget.

3B. If not using DriverSweeper, while in Safe Mode you're going to want to:

A. Remove any and all nVidia-related directories, including things that may be in the Temp folder(s).
B. Scour the Registry and remove anything "nVidia" you can find. As far as I know, it's all tagged with "nVidia" somewhere, so simply searching and removing the keys yourself should be sufficient. You want to be pretty careful doing this if you aren't used to altering the Registry, though.

4. The "one more thing" you want to check is in the Device Manager. Open it, click "View" up top, and select the "Show Hidden Devices" option. Now, look at your Display Adapters on the left. Do you see any grayed out items? If you do, remove them all. If all you see is the "Windows Default Display Adapter" you should be fine. Make sure anything you remove is under "Display Adapters" and nowhere else.

5. Reboot once more in to Safe Mode. Now comes the installation. Right-click on the driver installer and run it as an administrator. Once it loads up and you're able to start making selections, select a "Custom Install" and also tick the "Clean Install" box just for S&Gs. (At this point it should have nothing to clean, however.)

NOTE2: If you're unable to install the driver in Safe Mode for whatever reason, you should be okay to install it during a normal boot as long as you have no internet access, or nothing is going to try and update your drivers automatically. I can't express enough how much of a bad move this is.

6. When you're given the selection as to what to install, select ONLY the PhysX driver. The Display Driver will auto-install by default and as far as I know you can't untick it. Don't install the "HD Audio" crap nVidia offers you unless you absolutely have to have it and use it already.

7. Once done, you're free to boot regularly, with internet access. Just don't let anything update your driver for you if it offers to.

You should now be 100% free of any and all potential conflicts with your video driver. Old remnants left behind has been known to cause issues. The step involving the Device Manager is particularly important, as I had once found remnants for a GTX570 I had installed much, much later after having upgraded to a GTX760. (This is back in the day, mind you.)

While rare, it can and has happened. If you're a stickler for system stability like I am, this is unfortunately a necessary evil to slay when you either want to or are forced to (looking at you, Doom Eternal..) update your video driver. Sadly, I sincerely doubt nVidia's own installer follows these steps.

All of that mess aside, the only other thing I can offer is that it might be a 30-series card problem. I'm still seeing stuff all over the internet from folks with 30-series cards having issues with otherwise non-demanding games, or games the card can readily handle.

Despite the cost of those cards, almost every manufacturer is guilty of using cheap components in their construction. There are teardown videos from basically everyone (Jay, Linus, GamerNexus, etc.) that will show you specifically what they've done.

ONE LAST WARNING: if you have an EVGA "FTW3" 3090 in particular, be VERY CAREFUL of anything that might over-volt the card. Amazon's "New World" MMORPG has bricked a legion of these cards due to the aforementioned crap components that were used. (I believe related to an issue where frame limits were not in place, but has since been patched. It was with the game's beta specifically.)

ANYWAYS.. good luck.