Fair enough — it’s an invitation, not an obligation.
The <umb-imaging-thumbnail> element is the intended way to do this. There’s a complete example of it in a custom block view over in this thread — different extension type than yours, but the imaging part works exactly the same in a property editor. You hand the element a media GUID and it takes care of the rest — resolving the GUID to an actual servable URL, the imaging repository calls, and the HMAC signing — so your component never needs to know how any of that works.
That’s really two gaps in the documentation: the GUID-to-URL resolution story for backoffice extensions in general, and the HMAC layer on top of it, which is new enough from the recent security work that no older examples account for it. We’ll cover both — I’ll link the docs here once they’re up.