Is there a way to add all the Elastic repos to APT? It seem's daft to add individual sources each time.

I'm struggling with something similar. In the logstash repository, the version number is part of the path, like "deb". Our admins mirror it for me, but every time there's a new version I have to go and ask them to mirror the new one. While I'm excited every time elastic releases a new version, adding yet another repo isn't.
On, you explain that

We have split the Logstash package repositories by version into separate urls
to avoid accidental upgrades across major or minor versions. For all 1.5.x
releases use 1.5 as version number, for 1.4.x use 1.4, etc.

Still, I have that problem, so is it possible to offer an all version repo?

You can add the individual lines to the file, yes.

We are also working on normalising all the repos so they have the same path layout.