We also had the same problem. The trick we used was the following.
I don't know if this will work for all appstore content. We used it for: - community commons - email templates - SFTP connections - Flat file interface
They all work fine in Mendix 5
Many of these (such as Mx Model Reflection and Community Commons) actually have been updated for Mendix 5, but you'll have to hand-pick it in the App Store since there is currently no way to publish different versions for specific Mendix versions. You can find them under 'All versions' and/or by reading the documentation.
However, some of them still may only work in Mendix 4. Even though most migrations should be fairly simple, if you have no Java skills to get them to work you could consider running the demo in Mendix 4 instead.