It's not completely arbitrary, supporting another repository means they would also have to implement auth for the said repository, along with some other reasons I likely wasn't made aware of.

But that is a public image so you can use this bookmarklet in combination with an empty service -

https://bookmarklets.up.railway.app/image-deploy/

Thank you! (I had an error which was attached, but solved it by deploying the empty service first)

Yep you'd need to deploy that empty service otherwise there is technically no service.

Okay, I'm running into an issue - is there any way to make a template using a custom image?

There is not, what is the issue?

I want to make a template for people to oneclick deploy from this image - it seems like the best solution is to push to Docker Hub even though I don't love doing that

Yep, you can only make templates from the 3 registries you listed in your original post.You could also stick a Dockerfile in a repo with just -

FROM codeberg.org/reesericci/united:latest

Or build an image for GHCR.