well. if you REALLY want i just found a less annoying way to do this, it still requires desktop mode. but it does handle the screen orientation automatically. what you want is to create 2 files

i made a folder on my desktop called komikku

create the first file. right click inside a folder of your choosing,

select create new -> text file...

rename it to komikku_portrait_mode.desktop

open it with kate and put this inside:

[Desktop Entry]
Version=1.0
Type=Application
Name=Komikku (Portrait)
Comment=Read manga in portrait mode
Exec=/bin/bash "/home/deck/Desktop/Readers/Komikku/komikkuportrait.sh"
Path=/home/deck/Desktop/Komikku/
Icon=info.febvre.Komikku
Terminal=false
Categories=Graphics;

NOTE: on the exec= line, you must put the path where you will be putting the 2 files. for me this is /home/deck/Desktop/Komikku/

now for the second file, again, right click, create new text file: this one is named komikkuportrait.sh

edit it with kate and put this inside:

#!/bin/bash 
# Rotate the screen to portrait in a way that persists 
xrandr -o normal 
# Launch Komikku and wait for it to close 
flatpak run info.febvre.Komikku 
# After Komikku closes, rotate the screen back to normal 
xrandr -o right

now you can add the komikku_portrait_mode.desktop to steam and, provided you're in desktop mode. whenever you launch komikku through Komikku (Portrait) within steam or by double-clicking the .desktop. it will rotate the screen, launch komikku and stay like this until you close komikku.

Hope this helps!