Can't add keyframes in Audio Submix

I'm working with multiple audio tracks that are multi-track stems of a recorded song. In the past I've just nested these audio tracks and brought that nested seq into my main edit and added audio keyframes on that nest, but you can't see audio waveforms on the nest.

I tried creating a submix and assigned all those tracks in the Audio Track Mixer. But in my seq, the SubMix track is empty, so there's no option for adding keyframes. I can temporarily adjust levels in the Track Mixer, but the "Write" record option. I get that one main purpose of a submix is to apply effects to a whole group of tracks, but not having keyframe control over levels seems like an oversight. Or am I missing something? Thanks for any insight!