Long story short, RPG Maker MV is a barely working mess of a game engine. Unless a game has an official Linux port, it'll be barely playable as windows versions of RPGMMV games just don't function properly under proton/wine.
Just like the first game, Termina for some reason is marked as a "verified" Steam Deck game. It's framerate is terrible and barely holds about 20 FPS most of the time. Though it's much better than the first game, it can barely be called an acceptable performance.
Thanks to Savvy Kat user, the new solution for this problem has been found. Unlike the original solution, the new one works not only for the first, but also for the second game!

First of all, we need to download NW.js. To do so:

1) Open Google, type "nw js" and open the first link to appear in the search.


2) Open the "Downloads" tab.


3) Press the "Previous releases" button.


4) Find "v0.57.1" folder.


5) Download the "nwjs-v0.57.1-win-x64.zip" file.


After the download we need to extract the files from the archive to the game folder and do some configuring actions.


The path to the game may vary, the default one is:

/home/.steam/steam/steamapps/common/Fear & Hunger 2 Termina/

Having unpacked the files, we need to remove the original "Game.exe" file.


Having uninstalled the original executable file, we need to rename the "nw.exe" file to "Game.exe", it'll become the new launcher for the game.


And that's it! The game should work perfectly fine with this new method now and there should be no problems using it in Steam Deck's Gaming Mode!

As you can see, the game is initially opened in the window.


To get rid of the borderlines, we need to make the game to turn the fullscreen mode on. To do so, we need to bind the F4 button to one of the Steam Deck's buttons.

In order to do so:

1) Press the Steam button to open the overlay and press the "Controller settings" button.


2) There you should press the "Edit Layout" button.


3) There you should choose the button you want to bind F4 button to. I chose R4 back button for this.


4) Having chosen the button, open the configuration menu, choose the keyboard tab and choose F4 button to bind it to.


Now press the button you've chosen and that's it, the window's borderlines are gone!

That's it. Go on and enjoy the game now.

Thank you for reading, hope the guide was helpful. I'd appreciate it if you pressed the like button and added the guide to favorites.