Haven't looked at it in detail, but have you mentioned docker builds?
I see you wrote about mock, which is immensely useful for building RPM packages.
But docker seems well-suited for this task, for example:
<a href="https://github.com/alanfranz/docker-rpm-builder" rel="nofollow">https://github.com/alanfranz/docker-rpm-builder</a><p>What's your take on this?