PostgreSQL bad plan: the frequency of a tag depends on the project!
You are correct that there is no way to do this, and that CREATE STATISTICS only looks at arrays in their entirety, not their individual components.

Perhaps CREATE STATISTICS can be expanded in the future to do that, but someone would have to volunteer to implement it, and I am not aware of anyone working on such a thing.  

Note that even if "tag" were a scalar, CREATE STATISTICS wouldn't help you in versions 10 or 11, as the mcv type of statistics were not added until v12, and that is what you need. So presumably what you would need is a new type of statistics, call it arrray_mcv.