I'm not asking for the problem of key derivation to be solved by ElectricSQL per se, just wondering if it can handle E2E encrypted data in any way. That said, I disagree with how difficult it is: there are a lot of approaches that are not too ridiculous. For example, a simple approach to E2EE is to use a PAKE like SRP to do password authentication, then since the password is kept secret, you can use a normal KDF to derive a symmetric key from the password. From here, you can e.g. store symmetric and public key matter on a server or synced across your clients, encrypted using the password/KDF. If this sounds familiar, it's exactly how password managers work and the main downsides are that it requires password authentication (can't use any auth mechanism that doesn't somehow discretely convey a secret to the client) and the forgot password situation is more complicated (if you have no backups of the key, you can't recover any data. However, that's not an unreasonable compromise in exchange for E2EE.)

Just like TLS, it'd probably be bad if most people implemented SRP from scratch. That said, I did write my own implementation of SRP-6a (a variant of SRP based on the RSA cryptosystem) in TypeScript and I found it fairly simple to do. There are also PAKEs that provide even better security properties than SRP-6a, but good implementations of them are still lacking for now.

What Matrix does seems pretty simple, it just has a lot of moving parts and nuance; the underlying keys are essentially sent directly between clients after a key exchange is performed and verified out of band to ensure there is no eavesdropping and that the two clients are connected to who they think they are.

As far as I know, Signal sidesteps key management almost entirely and only allows one logged in device, and everything else must proxy through it. That's pretty lame, though I understand the stakes are very high to get it 'right' and keeping the moving parts to a minimum was likely high priority.