Microsoft added a custom UI option for MSIX Packages several years back, but the only reference to it was a web page How to create a custom App Installer UX – MSIX | Microsoft Learn that contained the following notice:

Because of this note, we have been ignoring this capability. I recently detected that the page had been updated, although …
TMEditX is the licensed and more capable tool for fixing up MSIX packages since it doesn’t have to depend on the Microsoft MSIX Packaging Tool to create the final package.
TMEditX is the licensed and more capable tool for fixing up MSIX packages since it doesn’t have to depend on the Microsoft MSIX Packaging Tool to create the final package.
PsfTooling is the free tool to work in conjunction with the Microsoft MSIX Packaging Tool.