I am running iOS 15.0 b3 but I can dictate both the addressee and message using share to Messages or Mail from Photo 🤷‍♂️. Just select the image, click the share icon, select the medium, touch where the text is to be inserted, then touch the microphone icon and dictate. Admittedly it can be a bit tricky getting the insertion point right, but that is more a factor of fat fingers than anything else.

FWIW I can't thumb type either.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein