There is an excellent guide detailing how to do this for the Question arcs here. This method may work for you, and I recommend trying it before this method. For some reason, I couldn't get it to work on the answer arcs, so I've made this guide for you all.

I recommend backing up your saves just in case, I am not responsible if you lose anything in this process.

You can do this by right clicking the game in your library > Properties > General > Steam Cloud
And then un-toggling the switch

On your Steam Deck, head to the discover store and search for "Synchthing GTK"

This program will allow you to manually sync folders across devices on your home network.

On your main computer, also download and install Syncthing from here[syncthing.net].

Make sure Syncthing is open on both computers before continuing.

On Syncthing-GTK, click on the box in the top left corner and open the web interface.

Here, you want to click on 'Add Remote Device' at the bottom left.
The nearby device should be reflected as a huge string of characters, click on it to add it and then give it a user friendly name.
You'll get a prompt on your computer asking if you want to add a new device, select 'Add Device'.

Both of your devices should be connected through Syncthing now.

Now, you want to add a new folder and set the path to where the Umineko saves are located. For me, this is the folder mysav, which the 07th mod uses.

Example Windows path: E:\SteamLibrary\steamapps\common\Umineko Chiru\mysav

Next, click on the sharing tab and click the device you wish to share with.

Back on your deck, you should have a prompt asking if you want to add the new folder added on the other computer, select Add.

You'll want to set the path of the saves on your Steam Deck under folder path.

Example linux path: /home/deck/.local/share/Steam/steamapps/common/Umineko Chiru/mysav/

So, to sync the saves between the two devices, you will have to open Synchthing on both computers whenever you wish to sync. You can add Syncthing-GTK as a non-Steam game on your Deck so you can access it in game mode for easy access.

Once that's done, you can easily test this by loading a save on one computer, saving in a random slot, and then accessing it on the other device to see if it changed. If all is well, you have manually setup cloud saving between your PC and Steam Deck. Enjoy!