Welcome to Microsoft Q&A! Thanks for posting a good question.
This is not possible currently, there is no way to create a shared build config and reference it. I understand you have been leveraging the workaround option, cloning existing build configs and using them, but mentioned it is incomplete. Today, the best option would be to clone existing build configs and using them.
This is great feedback, I would request you to post a feedback on this on Uservoice for other users to up-vote.
Having mentioned that, I have shared this feedback internally with our product engineering team.
Apologies for any inconvenience with this! We appreciate your feedback.