Yeah, the process needs another look at. The game is 70+ gb large which means two installations requires 140+ gb and that is assuming the store version works.
For anyone having issues with the store installation, you can use Tweaker on a steam installation to have it act as a switch between the store and steam versions, letting you link with just one installation.
First apply Tweaker to the steam version and select steam in the pop up. After tweaker is set up with steam, go to the drop down at the top and change it to North American MS. This will begin the process of setting it up to be able to run as a MS Store install with the flip of a switch. Make sure you get the cmd and powershell scripts from the arks-layer site and run them as well in your tweaker folder.
Once that it is done, start up the game in MS mode via tweaker, get your code, exit the game, switch to steam mode via tweaker, launch the game in steam mode and link your accounts.
This can also be done with an installation made purely with Tweaker.