Advanced roadmap, complex JQL

[Fred]

I'm trying to figure out query to populate an Advanced Roadmap. It's a brain bender!

I have a ticket ABC-123 that has linked issues of type 'has child epic(s)'.

These linked issues in turn have child links or actual children. This continues for a few levels, and crosses multiple projects.

Is there a way to traverse the whole thing and get the tickets from all the various levels? Chained queries perhaps?

Tickets may be added or removed in the future, so it'd likely need to be relative terms, not static ticket numbers, with the exception of the ultimate parent.