kleklok - Sorry for the delay. I downloaded the sourcecode for Thorium when I couldn't find an answer.
I do not see it set in their source; however, it appears they are using the %APPDATA% and %LOCALAPPDAT% (aka CSIDL_APPDATA and CSIDL_LOCAL_APPDATA) from Windows.
Therefore, the only way to move these, would be creating a "Junction Point" in Windows. This tells Windows "Anything stored on X -- move to Y." This is a totally normal, common activity in Windows, there is nothing to fear from doing this.
For example, imagine you want to move your entire APPDATA directory from the default location to D:\MY-NEW-DIR -- you would go into the DOS CMD windows and type this:
mklink /J "C:\Users\YOUR-USER-NAME\AppData" "D:\MY-NEW-DIR"
Or, if you only wanted to move Thorium alone (no other AppData directories):
mklink /J "C:\Users\YOUR-USER-NAME\AppData\Local\edrlab.thoriumreader-updater" "D:\MY-NEW-DIR\ThoriumReader-Updater"
mklink /J "C:\Users\YOUR-USER-NAME\AppData\Roaming\EDRLab.ThoriumReader" "D:\MY-NEW-DIR\ThoriumReader"
If you have any questions or difficulties, you can google "Create Juntion Point in Windows"; you will find this extremely helpful for the future!
[Login to see the link] - This also relates to the method I explained to you in the past, to save space on your C: drive 🙂