I think the best way is still to self-host your own encrypted file sharing service. A $5 a month VPS will be plenty for small-scale use. It’s not possible to be sure if the backend is actually secure, even if the code is open source, since you don’t know what modifications the server has. Even if it’s being encrypted on the client side, a malicious backend can just send you a backdoored version of the encryption code since you’re pulling JavaScript from it, and I doubt people are auditing every website they load every time. If you’re an organization especially, just host your own.
I think the best way is still to self-host your own encrypted file sharing service. A $5 a month VPS will be plenty for small-scale use. It’s not possible to be sure if the backend is actually secure, even if the code is open source, since you don’t know what modifications the server has. Even if it’s being encrypted on the client side, a malicious backend can just send you a backdoored version of the encryption code since you’re pulling JavaScript from it, and I doubt people are auditing every website they load every time. If you’re an organization especially, just host your own.