I can’t find any information on adding Delta update Artifacts when using mender-convert to create the img.
As the client requires the most robust OTA update system but we will only be making small application updates, delta updates seem the way to go. Is this not correct?
Deltas are possible on Debian/golden image-based deployments. However, they are a feature available on the Pro and Enterprise tiers: Features | Mender
So for small updates update-modules is the only option?
You can probably replicate a delta-ish mechanism as a custom Update Module.