Well somehow I've managed to fix it! Still not entirely sure what did it, but let me walk through the process.

I don't want to give anyone the idea that I have found THE solution for this. I feel like I might have been really lucky and did 'something' right. Let me start off by saying that I don't know how to fix it. All I can tell you are (by memory) the steps that I still remember I took that led me here.

So I did refund the add-on which didn't solve the problem.

After uninstalling and resetting the game for about 5 times, I figured there might be some stuff left on the disk after uninstalling so what I did was I searched the PC for 'forza' using Everything Search (Google that). This is basically a fast search tool for Windows. Then I deleted all the things I could with Forza in it. Maps, files, images. Some things couldn't be removed. Especially the files in a folder called WindowsApps.

What I did was I right clicked this folder and went to properties > Security > Advanced.

This screen showed up. I clicked Change:

The next window showed where I clicked Advanced:

Another new window opened. I clicked 'Find Now' and double clicked on my Windows Account:

The next screen showed up with my name in it, I clicked OK and on the next screen also OK:

This gave me the rights to delete the contents from the WindowsApps folder. I searched for anything that had to do with Forza (sunrisegame) or the DLC or Forza Cars and deleted it.

Then I went into the Registery and searched for 'forza' I tried to delete anything I came across referencing to Forza in the register. Then I restarted.

Let me be very clear here. I felt like I had nothing left to lose and if this would break my Windows installation then so be it, but usually just removing stuff from the Registery isn't a great idea if you don't know 100% sure what you're doing. ALSO I'm not sure if this fixed the game, but if you're still curious then please read on...

I restarted my PC after doing all this and tried downloading Forza again, but this time on another disk just to be sure. This did not work. I'm pretty sure I broke something, making it seem like the game was installed, but it actually wasn't. Anyway I tried many things now since I couldn't correctly install the game anymore. I tried resetting the windows store and the Xbox Beta app and it didn't seem to work. At some point I deleted everything again and tried to reinstall the game again. To my surprise the installation process only took seconds (For a 74 Gb download) and I couldn't immediately find why. After a while I realized that the game wasn't uninstalled correctly. All the data from the installation was still on the disk so I manually deleted it. Now I reinstalled the game yet again. Took several hours to download. I picked the old disk where I always installed the game as source for the game and waited for it to be installed. Suddenly the game worked...

These are the things I love/hate so much about these things. Ofcourse I am very happy that the game is working right now, but I don't know what actually fixed it and what the actual issue was. I feel like the issue is that the game makes a Registery entry somewhere or maybe adds some files that screw up the installation. Only by deleting those files/registery keys you get to play again, but then again, I'm no expert.

I really hope this is helpful for you. I still have the escalation ticket with the support and I will await updates from them too so if I hear anything new I will let you know!