It does not make sense as you state - "the mariadb source type is unknown" I suggest you export the mariadb schemas. then based on the schema you can import the schema into mysql. Following that you can export the data from mariadb (use a csv if you have to) and then import the same file into mysql.

Another point, why can you not export the mariadb to a dump and then import into mysql - they are very compatible? Or is there some reason about the data that prevents this. Suggestion: pull the mariadb down locally - do the conversion offline; the upload and impart into mysql. 90GB is a small dataset. It probably is faster!

Key tip: focus on getting the mariadb schema