I don't know how to control opacity with speed — not even with Lazy Nezumy Pro — but in Brush Settings > Transfer set control to Fade. You'll need to set the Fade value according to your expected brush stroke.
But what you can do with LNP is invert pen pressure controls size so that the harder you press the small the stroke.
Thanks for your suggestions. Unfortunately, not quite what I'm looking for.
I have discovered that Adobe Fresco has a velocity slider on some tools that allows control over flow, and it would be ideal if that same feature was available on Photoshop tools as well. I suppose I need to suggest it on Adobe's feedback system.