Based on this thread it’s the deduplication that requires a lot of RAM.

See also: https://wiki.freebsd.org/ZFSTuningGuide

Edit: from my understand the pool shouldn’t become inaccessible tho and only get slow. So there might be another issue.

Edit2: here’s a guide to check whether your system is limited by zfs’ memory consumption: https://github.com/openzfs/zfs/issues/10251

Then why didn’t you contact the devs or opened a bug report on GitHub?

Additionally this isn’t the community where this needs to be addressed. Either contact the admins or open an issue on GitHub.

Greek and Roman god names all the way!

For syncing I use Syncthing. It’s open-source as well and syncs two/multiple devices without the need for cloud-storage