How can I filter content with conditional taxonomy terms?
You can use https://www.drupal.org/project/shs in combination with views.

From the project page:

Views (exposed filter)


add a new filter using the field set-up as "Simple hierarchical select" or
use "Content: Has taxonomy terms (with depth; Simple hierarchical select)"
as a new filter
use "Simple hierarchical select" as selection type
select "Expose this filter to visitors, to allow them to change it"
enjoy ;)