Nope - you have to encrypt them to send them to WhatsApp. Why could you not encrypt them in the client and then send that encrypted message over the bridge, preserving E2E?

Because that's not how this works. The bridge has to have the unencrypted text, because it's the bridge that is communicating with WhatsApp/Signal/IRC/whatever. The client isn't the bridge, it's not communicating directly with WhatsApp, it's just communicating with a Matrix Bridge (over an encrypted channel) to a Synapse server you don't control.

You'd need WhatsApp's collaboration for this, which I'm going to go out on a limb and suggest that the bridge operator doesn't have.

There are two encrypted channels: client<->bridge, and bridge<->WhatsApp. The bridge can read the decrypted text, and the comment I replied to is a lie.