MSIX app issues: failing to include exe & dll

Question

After a disastrous app (Desktop App Converter) to bring desktop programs to the Microsoft Store Microsoft seems to have found another way to discourage developers to do so: the MSIX package app. I think I have finally solved the issues but I wonder why it doesn't work as it should + I hope this is useful for others. For the previous package I used a .MSI installation package created by Advanced Installer, which I had to use for the Desktop App Converter. The .MSI installs my program and files fine. When I use that .MSI as input, 2 things went wrong:

Process msiexec.exe failed with exit code 1638 at Microsoft.Msix.Utils.ProcessRunner.ProcessRunnerBase.ValidateExitCode(Int32[] validCodes) and a window Installer fails to run with error code 0x80131500

Of course (as usual) there is no clear documentation of what error 1638 is exactly or how to prevent it, nor any usable answer on the Microsoft forums. But after some trial and error I found out that this error could be prevented to first deinstall my program via Programs and Features. The .msix file was now successfully created.

There is no way to install & test it as it was unsigned (and, as far as I understand, that is how it should be to post it as an update for my program in the Store).

However, I could open the .msix file with the MSIX app and inspect the contents. This showed that the program's .exe and none of the .DLL's, although clearly included in the working Advanced Installer created .MSI package, were present in the MSIX. I finally solved that by creating a setup.exe with InnoSetup: now everything, including Store logo's, seem to be in the msix package. I do wonder however why the equally working .msi installer file did not output a complete .msix file. Next phase is to post the msix file to the Store. Let's see how many problems that will give.

PS: why can't I enter MISX as tag?