When editing a post on the wordpress admin I need to be able to, with relative ease, find and select a taxonomy from a list of maybe 300 - 500 taxonomies. The taxonomies will be assigned a parent taxonomy, which will total about 20 overall. I'm looking for either an autocomplete type deal or having an accordion that expands when the parents are clicked. Can it be done / is there an existing plugin?
To answer your question, yes it can be done, and there are existing plugins. I was able to find one and have linked to it below
https://wordpress.org/plugins/admin-taxonomy-autocomplete/screenshots/
If the plugin listed above does not meet your needs, one will have to be created (it is possible). As David commented, you will either have to learn to code or hire someone to build it for you.