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When you set up Nextcloud, it has a wizard prompting you for "apps" to install. Can't remember what the choices are exactly, but there's a "simple" choice that is just file sharing.

Yes, you can disable the internal apps it ships with if you don't want to use them and just not install any third-party ones either. Only caveat is during one upgrade I noticed some of them had been re-enabled so now I make sure to check each time.

Probably the most simple way would be to rent S3 compatible storage at Lidonde/DO and use client like https://mountainduck.io to mount it in your system.