The support article Change your Apple Account country or region - Apple Support describes how to change your country setting on everything using your Apple Account, and the requirements and results of doing this.
"Redownload your apps, music, movies, TV shows, and books on any devices that you might use to play or read them in the future. Some types of content might not be available in your new country or region. (Learn what types of content and services are available in your new country or region. - Availability of Apple Media Services - Apple Support )"
As Niel said, you may not be able to redownload purchases even if you don't change country if a distributor removes them from the store. Download everything and keep a backup copy if you value them. Buy an external drive (or a pair) if you don't have room on your computer. I consider anything I store with Apple online as potentially ephemeral.
"To make sure that you can access your previous purchases in your new country or region, redownload your previous purchases on every device on which you might want to view the purchase before you change your country or region." Subscription services have to be canceled and started again.
You are changing Apple services completely so I would not presume that your iCloud data is rolled over from one service to another.
I don't know how to check for app availability by region other than finding the URL for the app on the App Store and then forcing it to use a different country reference. E.g.,
My default in the USA: https://apps.apple.com/us/app/maps-me-offline-maps-gps-nav/id510623322
German version: https://apps.apple.com/de/app/maps-me-offline-maps-gps-nav/id510623322
Note the "us" and "de"
Any iOS apps on your devices now will stay put, but if, for example, you buy a new iPhone and an app you value is only available in your old country, you won't be able to redownload it onto your new iPhone in your new country.
