Hi Michael,
I agree with your points, most information is selective or incomplete. So we may try another way from ammunition consumption and casualties (only those killed or wounded).
At Essling (or Aspern), the French army fired 24300 rounds and 1616000 cartridges (Buat, Étude critique d'histoire militaire 1809, vol.1, p. 271.)
At Wagram, the French army fired 71129 rounds and 724260 cartridges (Journal des armes spéciales et de l'état-major, Serie 3, Tome 1, No. 5, p. 426. https://books.google.com/books?id=Sxb5Kn2jaccC&pg=RA1-PA426)

If we assume the quality of both sides did not change sharply (it did change somehow), we may establish a relationship between the French ammunition consumption and killed/wounded in the Austrian side.
If we accept 21000 Austrians killed or wounded at Essling (or Aspern) and 24000 for Wagram and assume the share of cold weapon is rather small, then we may get one casualty for every 4.13 rounds or 106.91 cartridges.

Anyway, it's only an amateur try, I hope someone would do more research on this filed.