I have used conversations (and derivatives), gajim, dino, converse.js, and movim to have omemo encrypted xmpp conversations; and I talk to people on Siskin and have used profanity in the past. Seems to work properly, unless I'm missing something.

Before XEP-0459, every client implement their own subset of standard and seldom talks with clients that implement a differnt subset.

Now try to make a voice and/or video call.

I do/have, with conversations, siskin, movim, and I think dino. Can you expand on your experiences?

gajim, pidgin, dino... none of them worked when I tried to call from my desktop to my Conversations' app on Android.

Does any Matrix client support voice/video calls other than Element (including Hydrogen as it shares Element Web's codebase)?

Fluffychat supports it too. Check out https://matrix.org/ecosystem/clients/, you can click the clients for a feature list.

Hydrogen does not share a single line of Element Web’s codebase.

I must have got it confused with another client then. Wasn't there another lightweight one made by Element-the-company that reuses the react SDK?

nope.