I stopped using it since they keep not updating the electron version they are using. It took them forever to update after electron 28 went EOL and they updated to a version that would go EOL about a week later, which they have been on ever since..
Edit: They just bumped to a supported version, but only in master. It's still unknown when a release with a supported electron version is coming.
First of all, something not being free (as in gratis) does not mean it is proprietary per se.
Second of all, your reading comprehension failed you again:
However, if you cannot, or do not want to pay, you can click the button at the bottom to use DeArrow for free. No worries if you can't or don't want to pay :)
I don't think they meant forcing themselves because their RAM would fill up, but because their stuff would be gone after rebooting if they didn't move it.
I'm not using it at all, I use the *DAV functionality of my paid mail provider.
I did research on what solution I would use alternatively – with requirements very similar to the ones you proposed – and EteSync checked all the boxes, which is why I recommended it.
I set up an instance once, and it seemed to work just fine, but I haven't thoroughly evaluated it.
I stopped using it since they keep not updating the electron version they are using. It took them forever to update after electron 28 went EOL and they updated to a version that would go EOL about a week later, which they have been on ever since..
Edit: They just bumped to a supported version, but only in master. It's still unknown when a release with a supported electron version is coming.