@iceystreeeet I am wondering if this might help.
When you intitally run the installer and get to that content manager page to select the download directory for packages, instead of selecting the default location suggested, instead use a folder like C:\MSFS.
Windows has a problem, sometimes, with really long path names, and changing the installation directory for packages reduces the likelihood of this being an issue.
There are details here on how you can move your existing downloaded packages to such a folder, and continue the installation in the new folder.
I really hope you get it sorted.