Thanks Ronny!
I’ve actually forgot to add one CRUCIAL point – I know I can export to multitrack MOV with H264 compression (i.e. your first tip).
BUT!
The resulting MOV file is largely incompatible with most video players (= something a client would actually use), as well as typical web video servers. It plays correctly (with all audio tracks) in Quicktime Player and Quicklook, I can import it to NLEs, but VLC, Youtube, mplayerx and every other app including Adobe Media Encoder just plays the first audio track.
In your case that doesn’t matter as there is only one audio track (a proper mixdown), but sometimes I don’t have or need that, I just need to export the thing with a specific language VO and probably a couple of music and SFX tracks. And that’s still seems impossible without going to ProRes first and reencoding from there.