How to update Craft CMS/plugins without downtime?
I think the best solution would be to integrate a CI with "Zero downtime deployment". 

You may have a look at this article:
https://craftquest.io/articles/what-are-zero-downtime-atomic-deployments

Or have a look at this deploy plugin:
https://github.com/elfacht/craft-deploy

Or just google for "zero downtime deployment".