Been struggling a lot but finally managed to do it. Get your FH6 build from your trusted source. If you can find a Steam dump, the better. Look for the DenuvOwO crack on cs/rin. Once you put the files from the crack into the root folder of FH6, you need to do the following inside your steam_settings directory: Open configs.user.ini and add the following lines:

[main::connectivity]
disable_lan_only=1
offline=0

[main::overrides]
disable_all_dlc=0

Then go to your configs.app.ini and make sure this parameter is set:

unlock_all=1

After that, you need to change the steam_appid.txt located in both your steam_settings folder and the main game root folder to:

480

Step 1: Add the Game to Steam Natively

1. Switch your Steam Deck to Desktop Mode.
2. Open Steam > Add a Game > Select Add a Non-Steam Game...
3. Browse to your executable path (even if it's installed on an external microSD card) and add it.
4. Right-click the game in your library > Go to Properties.
5. Paste this exact string into the Launch Options box:SteamAppId=480 VKD3D_CONFIG=no_upload_hvv,single_queue PROTON_VKD3D_HEAP=1 %command%

Step 2: Inject the Registry Overrides via Protontricks (Most Important Part!)

If you skip this step, the DLCs will not work. Instead of using destructive global environment launch arguments that crash modern Proton, we use Protontricks to burn the DLL overrides directly into the game's isolated Windows registry hive.

1. Open Protontricks from your Steam Deck's desktop application menu.
2. Select your game from the list (it will be listed alongside its random 10-digit Non-Steam AppID) and click OK.
3. Choose Select the default wineprefix > Click OK.
4. Select Run winecfg > Click OK.
5. When the Wine configuration window opens, navigate to the Libraries tab.
6. Under New override for library, manually type version and click Add.
7. Repeat this step to add steam_api64 and steamclient64 (if applicable to your specific bundle).
8. Verify that all three show up in the list as (native, builtin). Click Apply, click OK, and close the window.

Step 3: Install Missing Windows Subsystems

Many modern game pass/retail bypass hooks rely on specific automated Windows authentication components that aren't enabled in a default blank Proton prefix. This ensures the custom version.dll can actually compile its memory hooks when the game initializes.

1. Go back to the main Protontricks menu for your game.
2. Choose Install a Windows DLL or component > Click OK.
3. Scroll down and check the boxes for vcrun2022 (Visual C++ 2015-2022 Runtimes) and d3dcompiler_47.
4. Click OK and let the deployment scripts execute completely.

Edit: The Launch Variable "VKD3D_CONFIG=no_upload_hvv,single_queue PROTON_VKD3D_HEAP=1 %command%" Can cause some textures artifacting. So use this one instead:

SteamAppId=480 PROTON_VKD3D_HEAP=1 %command%

Also added Step 3 in the end too. Forgot about that part.
Sorry.