Maybe this reply is so very late. But, I just want to share my experience.
The app I use to published my build with Lazarus or Delphi apps is MSIX Packaging Tool. Aside from registration, a simple method I usually use to create an MSIX file could be done in 2 steps. First is creating installation with Inno Setup. Then let MSIX Packaging Tool records how the setup runs to produce an MSIX file.
The main glitch is the lack of code or library to use Windows Store API. Up to this date I still cannot create or find the interface for Lazarus or old Delphi. For me, the important functions are checking license and add-ons. If we are going to publish free apps (and without add-on), I guess MSIX Packaging Tool is quite enough.
And yes, newer Delphi has TWindowsStore component for this issue.