It's important to note though that their client is not open-source, so if one goes through all that trouble to use end-to-end encryption, it seems a bit unsatisfactory to me to then trust this company to actually keep the private keys on my machines (and encrypt things correctly).

Personally I used syncthing which doesn't do encryption but also only uses my own devices, so I can keep the data on my machines at all times.

In the past I used seafile which does support encryption (and it's self-hostable): https://www.seafile.com/en/home/

Even though it's proprietary, Resilio Sync does encrypted peer to peer sync, plus it allow encryption-only nodes. These contribute bandwidth, but cannot decrypt the data.