File Sync App Recommendations for 2020

Like many people here on HN, lately I've become unhappy with Google and their attitude and actions. I've also become unhappy with Google Drive and the way its associated "Backup and Sync" tool works.

May I please have some recommendations for a replacement file sync tool? It should fill these requirements:

    Cross-platform: Linux, Windows 10, and Mac OS X.
    Stable and fast: No huge inefficient UI taking up memory.
    Multiple locations: i.e. "cloud" based.
    Pricing equivalent to my current $10/month for 2TB.
    "Good" corporate behaviour: open about security, no selling info, no bait and switch etc.
    Works OK with more than a million files.

An open source solution using something like S3 would be OK, but I'm not up to date on the latest offerings in that space.

It would be great if this tool would also allow multiple accounts so my wife and I can share space and files.

My reason for moving is that Google's file sync/backup tool has multiple "containers" such that if you sync folders outside of your "GoogleDrive" folder, that folder goes into its own container which isn't accessible from any other computer. You can access the files through the web interface, but you can't sync them to multiple computers. (There's a feature to "include folder in drive" from these containers, but they don't allow top-level folders, so you have to jump through silly hoops to get it to work, plus Google may just decide to remove that feature one day)