

Not using GE-Proton9-2 will lead to pre-rendered cutscenes having showstopper issues (e.g. non-progressable black screens, "TV color test" screens, etc). This is the most up-to-date Proton/GE that fully works in my tests.

Game itself runs flawlessly, but it took me a while to get the cutscenes to work (see below)
vk_pro %COMMAND%
None of the pre-rendered cutscenes (including the opening credits) played correctly, instead showing the dreaded test screen indicating missing codecs
How I managed to get the cutscenes to play on Arch LInux after a long session of trial and error (on Arch Linux):
I first had to install the proprietary AMD drivers and amd-vulkan-prefixes in order to be able to switch to the new proprietary drivers (both of these steps might differ, depending on your distro).
Afterwards I downloaded and installed the specific version 7-33 of GE-Proton and enabled that one in the game's compatibility options in Steam.
Last, I changed the launch options, also in the game's properties, to vk_pro %COMMAND%
in order to start using the proprietary drivers.
This was the only way I could get the cutscenes to work on my system. Newer versions of GE didn't have any effect (except for crashing my graphics driver once and forcing me to hard-reboot) and mf-install
just changed the test screen that would show up instead of the cutscenes to a black screen.

GST_PLUGIN_FEATURE_RANK=protonaudioconverterbin:NONE %command%
Fixed with GST_PLUGIN_FEATURE_RANK=protonaudioconverterbin:NONE %command%
Game goes to windowed when start because of pop up
To have cutscenes working properly use GE-Proton7-28 with the command GST_PLUGIN_FEATURE_RANK=protonaudioconverterbin:NONE %command%

With proton 9 cutscenes work as intended and you can even increase the graphics to the high preset and i would recommend to turn the shadows back to medium. 4h into it and having a blast on the deck.

some very unique and interesting bugs, but nothing that will get in the way of your experience too much
sound effects in some cutscenes are missing, all voices and music are fine but everything else like the clashing of swords is simply missing, this is not normal behaviour as indicated by videos of other people's playthroughs on Youtube
the intro cutscene when you boot up the game does not work, i am unsure why. it displays the typical TV test card with static in the lower right side corner
in windowed mode, if the game is in focus and the mouse cursor is not within the game window the framerate will drop to around 10-20 FPS after around 30 seconds
if you are returning to this game then you will be less affected by these bugs, you also may not have these bugs!

Standard Proton has the issue of prerendered cutscenes not loading. Switched to latest GE and everything works flawlessly.
Works (almost) as intended.
Pre-rendered cutscene movies won't work. Interestingly stage intros and ingame cutscenes are working, so you won't miss out much story-wise.
Amazingly stable experience. Aside from a couple of broken cutscenes, it's about as good as it gets.
Without using Proton GE the game has issues with videos. Regular Proton seems to cause the game to hang before you get to the main menu. Experimental causes the logos at the start to give a test pattern instead of loading properly but does manage to get to the menu at least.
I am happy to announce this game now works on my compuer with no issues. No launch commands needed. Just set to Proton Experimental
Had to wait after the "save game" warning at first launch for a minute before the next menu will load.
While framerate does not have problem, the gameplay is shaky when running on Wayland, but x11 doesn't have problem. Mine is a 144Hz monitor, seems to be VSync on Wayland issue
When opens the game, it shows black screen for a while with the "Program not responded" dialog and ask you if you wants to terminate or wait the game to run. It takes some patience (around 10-20 seconds) and the KT logo will show, game will behave like native afterwards

Game window would not launch if I tried to use PROTON_USE_WINED3D=1 %command%
I would get an initial menu that would tell me the game saves data automatically, and to not shut off the system. After that the game would freeze up. I assume it is supposed to play a media file here. I used MF-Install as another user had suggested, and it sounds like I am at some kind of main menu screen but the window is entirely black. I downloaded and tested on the trial version of the game, which does have a different STEAM ID but I do not think buying the game to then test it would yield much of a difference.

Well, there's an autosave setting...
Could see that there's autosave and i shouldn't close the game while it's saving. Then it goes dark.
Unable to proceed to main menu for current proton experimental & proton ge
No video playback.
For Arch Linux, I noticed that black screen bugs would occur if vulkan-amdgpu-pro and also possibly the amf-amdgpu-pro packages were NOT installed, so please make sure to install those packages. After I installed those packages (I didn't mare any more changes), I could play the game without any additional tinkering.
The game runs perfectly(with some tweaks), the only problem is the cutscenes that don't play.
To run this game you need to use mf-install: https://github.com/z0z0z/mf-install

It crashes on the first screen that the game throws up
Tried multiple versions including GE, none worked.

No tweaks are needed, on contrast, you shouldn't add unnecessary tweaks for this game. This is only the demo version though.
No tweaks are needed, on contrary if you try to force the game to use Vulkan, only a black screen will be displayed. Some small stutters when the game just started, the anti aliasing seems not to work too well. Also, no video playbacks. The game is solid at 60fps with high settings though.

Game would boot up to a prompt and proceed to a black screen on the first loading screen.
Tried with Proton experimental down to 5.0-10

Black screen
The game even displayed the first menu, but after that the game stops on a black screen

Only a black screen. You could hear the sound effects, music, etc but nothing else.
I tried most latest versions of Proton with no luck: DXVK_HUD=fps DXVK_HUD=fps VK_ICD_FILENAMES=/usr/share/vulkan/icd.d/amd_pro_icd32.json:/usr/share/vulkan/icd.d/amd_pro_icd64.json %command% %command%