Dropbox-like file sync software decoupled from storage provider

Is there such tool available? My wishlist is:

  * Realtime two-way sync (no CLI tools required to run manually).
  * "Serverless" in the meaning that no software is required to run and manage on the server side (i.e. Nextcloud doesn't meet this)
  * Proper client-side encryption, storage and transit always encrypted
  * "Bring your own cloud storage" - ability to use mine S3/B2/etc credentials.
  * MacOS and Linux clients available
  * Syncing to a cloud location (not requiring my two computers to be online at the same time)
  * Bonus points for having an open code, but a closed source from a reputable company might work for me as well.

I only found GoodSync, but it seems very niche, not popular with the IT crowd and I'm not sure how trustworthy it is. I know that JungleDisk used to work this way years ago, but they don't seem to include "sync" in their feature list today and seems they pivoted towards backup solutions.

Is there a solution meeting these criteria? What do privacy-oriented folks use to sync their files across machines?