Desktop Refresh Error: Query "references other queries or steps ... "

Well, the ticket with Microsoft wasn't especially helpful. Essentially the answer I got was that this issue was due to an architectural constraint in Power BI Desktop and that it wasn't worth the re-architechting effort to fix it. They pointed me at this blog post (https://excelguru.ca/power-query-errors-please-rebuild-this-data-combination/), which at least somewhat explains the literal meaning of the problem and outlines some steps to reorganize queries to avoid the problem.

TLDR: you will only get this problem if a single query references a data source directly (e.g., the original source in the query is the database, spreadsheet, etc.) and then you join in another query defined elsewhere in the dataset. If instead, you load both data sources into intermediary queries and then join those queries in a third query, you won't get the error.

There are some downsides to this, notably that you will lose any query folding after the join, even if the two original queries are pulling from the same database.

Frankly, I'm probably just going to go with Sophie's solution and ignore the problem haha.