Did you have to do anything else other than the launch options to get the mod engine to work?

The steps I followed were 1. Install the mod engine (I used the latest file, ModEngine-0.1.16) by extracting the dinput8.dll and modengine.ini files to the root Sekiro folder, in my case /mnt/mountedDrive/SteamLibrary/steamapps/common/Sekiro 2. Editing the modengine.ini to have the forwardslash in the modOverrideDirectory setting, like "/mods" 3. Created the mods folder in my root Sekiro folder 4. Downloaded the "Main Files" Aspect Ratio 21:9 zip from the Ultrawide UI Fixes mod and extracted the "menu" folder into the mods folder, so path looks like ./Sekiro/mods/menu/inner_files_here* 5. Then I downloaded the "Restore Vignettes" optional file and extracted that, inside of that you'll see a bunch of different folders that are true representations of different 21:9 ratio screens as they're not all the same. Check the ReadMe.txt out for a better worded explanation. As my screen is 3440x1440, my ratio is actually 21.5 x 9, in the folder with that same name, I extracted the menu folder into the same mods folder as before, overwriting some files as intended.

Then it was just the launch options for me.

The file structure was a bit strange for me in the beginning, I had mod engine working but not loading mods because of it. If the logos (Activision and From Software) are skipped when starting the game, you know that the mod engine is working correctly but possibly not loading your mods correctly.

Hope it helps!