What apps in Google's ecosystem have the "all files" permission? Google Drive certainly doesn't. The "upload" button on GDrive prompts you to select a file just like NextCloud does.

The "sync just one folder" functionality exists in SAF without any high-risk permissions. Migration of existing profiles may be a pain (as the user would need to grant permission on the folder when switching to the new API).

Synchronisation of the entire virtual storage, the download folder, or any extra folders vendors like Samsung might've added to the blacklist, isn't possible with the new API, but it's also not possible with Google's own services. The DMA only requires Google not to be put in a special position; as long as they don't offer such a feature, they don't need to offer it to NextCloud.