Uploading VeraCrypt file via browser

I think for your use-case age, GnuPG (symmetric encryption mode), Kryptor, Picrocrypt or Picrocrypt-NG would be a better way of doing it as it seems that your goal is just do encrypt your files before uploading them. These tools do exactly that, and VeraCrypt is not made for that. Especially if you need „absolute certainty"