Hi Tim
This comes up from time to time, and we don't really have a good solution for it.
Here's what I would do.
If you have a hierarchical taxonomy you can only have a single form input for that taxonomy. You can't have one input for the top level, a second input for the second level etc. That's how forms work (and the filter controls on a View are a form).
So you need to fake it. You need one actual input that has all the submitted terms, which you hide, and you add your own inputs which are not meaningful—they will be discarded when submitted—but which you use to set the terms on the actual input.
You basically need an associative array of the complete hierarchy of your taxonomy. You use the first level of the array to make your top-level fake input.
You use event listeners to react to changes in that select input to populate the second level fake input, which in turn has event listeners to respond to changes in it to populate a third level fake input. For each change, at any level, the hidden real input is updated accordingly.
The trickiest part is creating the associative array that maps your taxonomy in the first place.
If you include the filter as a multi-select, for example, then all the data is there (see screenshot). You'd need to use jQuery to cycle over the select options and use the number of hyphen prefixes to determine the level of the term in the hierarchy.
If you are more comfortable with PHP you could build up the array of terms in PHP and then use wp_localize_script to make that array available to JS on the front-end.
Something like: https://wordpress.stackexchange.com/a/99516/35739
And see enlace oculto
If you have some programming ability and want to have a go then let me know if you get stuck on some detail.
If you are at a loss as to where to start, though, then you'd need to get a developer to create this for you. You could try the list of Toolset Contractors here: https://toolset.com/contractors/