Seems like a market opportunity exists for this especially with Apple cutting external drive support for third party sync tools including DropBox.

You can still do this, but Dropbox can’t use the File Provider API for that yet, so the experience won’t be quite as integrated as it is with Dropbox for macOS on File Provider. See https://help.dropbox.com/installs/dropbox-for-macos-support for more.

> Insync sucks...

FWIW, i've been using Insync on Linux since it went online (because Google never released a Linux-native client). Aside from one massive screw-up on their part about 8 or 10 years ago (where they automatically converted all of my 100+ gdocs-format files to MS office and deleted the originals), i've not had any issues with them. (In that one particular case the deleted gdocs were all in the trash bin, so could be recovered. Nothing was lost, it was just a huge pain in the butt.)

> Aside from one massive screw-up on their part about 8 or 10 years ago (where they automatically converted all of my 100+ gdocs-format files to MS office and deleted the originals)

Why. Just why. How does that shit ever happen in a public release?

You probably want syncthing

You might want to try Unison: https://github.com/bcpierce00/unison

I've been using it to great effect for over 10 years on a daily basis.

But absolutely 100% remember to block automatic updates because even minor-minor version updates change the protocol (and even different versions of the ocaml compiler with the same version of unison source can mismatch.)

This pain has always stopped me using Unison whenever I give it another go (and it's been like this since, what, 2005? with no sign of them stabilising the protocol over major versions.)

The recent updates have stabilised things a while lot with nice features like atomic updates.

Syncthing maybe?

I have bi-dir sync from/to two computers, one running Windows 10, the other Linux, using Google Drive as buffer. All data are encrypted client-side because I don't want Google to know my business. But the sync is every 30 min, not immediate. I have no fancy GUI over it, like synchthing etc. just plain CLI command triggered by cron and the task scheduled.