Hi @Andrew Blyth , Welcome to Microsoft Q&A forum and thanks for reaching out here.
- Not sure if you have got a chance to go through this article by a community volunteer. See if that is what you are looking for : DATA DICTIONARY USING DMV QUERIES. This utilizes DMV queries to build the data dictionary.
- You can explore Tabular Model Explorer which helps you navigate metadata objects in tabular models.
- Since you are open for third party or open source tools. Please try the below:
BISM Normalizer
Extract Model Metadata
Hope this helps.