Here's my journey on this disaster of a launch.
First up, I advice you to uninstall and have a clear slate for this, or you can try this out as if. At this point, it could work either way.
It's obvious right now but just in case I will explain how to make the game show up on the Store to be able to download it.
Now, press Windows+I to get the settings for windows. Click on Time & Language ► Region ► Country or Region. Select United States on the drop down menu and open up the MS Store.
Moving on, let's tell Windows where to install the app and where to look for it after booting up. Spoilers, Windows still doesn't remember where you installed the game.
Press Windows+I to bring up the settings panel. Click on System ► Storage ► Change where new content is saved. Click on the drop down menu in "New Apps will save to:", select the drive where you are going to install PSO2 and Apply.
This is important, and will come back to this in a bit.
Now you can install the game if you haven't.
Start PSO2 and the launcher will Check for Files and it's gonna take a while (1 hour in my case), when it finishes that step it will start to download missing files (You can use PSO2 Tweaker to download the full game since its a LOT faster than the snail paced OFFICIAL launcher). The tweaker didn't do much aside from downloading because even if it managed to launch the game, there was ALWAYS something wrong with it. So I just use the official launcher. Allright, that's done.
At this point you should be able to play normally. That is until you "Turn it off and on again".
The next time you startup your PC (or maybe even after a reboot), PSO2's shortcut will show up greyed out and also it will tell you on the store to reinstall. Don't panic. Curse SEGA and MICROSOFT for it. Remember that thing I told you about that was important? Yep, do that again but this time, change the selected Drive (Installed game) to a different one, apply and then change it back to the original (the one where you installed the game). This should fix the issue. You can start the game again from the shortcut and, regretably, the launcher will check the files, (that I bet Windows itself deleted/corrupted and it took me 30+ minutes to download) then download anything missing/corrupted and after all that mess, you can finally play the game. Hopefully...
I've left the worse thing for last, you have to do this after every reboot/startup. Though the bright side is that you dont have to download 70GB every time.