You seem to be asking 2 questions here:
Restricting Terms: You can edit the entity reference field's Reference method to use a new custom view that filters only the criteria you're looking for.
Parent/Child Widget: The Hierarchical Select did have this widget functionality in D7, in D8 there are a few modules out there, like Simple hierarchical select &...
I found this thread looking for the same functionality, and wanted to post another option I've found:
the Rabbit Hole module
(35k reported installs as of 2020-03), which allows to disable the default pages for nodes, terms, and other entities, with a variety of options, including per bundle and per entity.
I figured out the solution.
The value stored in the config/sync is not the tid of the taxonomy term, but the uuid. Because of this, I can just change the value of uuid column in taxonomy_term_data row of the dev's term to that of the stage's server without affecting the tid column.