Thanks for the tutorial, TSD! Just one note:
Exactly. Unfortunately your screenshot doesn’t show how you do it 
But it’s actually pretty simple, just enter Port=3307 under Advanced Connection Options.
In case you wonder why MariaDB is running on port 3307 anyway - it’s because Synology allows you to install and run the original MySQL server and MariaDB side by side (something you would not do on a regular Linux server – most Linux distributions now ship with MariaDB instead of MySQL, and actually use MariaDB when you install the mysql-server package on your server). So it uses a different default port for MariaDB to avoid conflicts.
Another thing, please note that it might not be legal to use the libmysql.dll from the original MySQL package unless you have a commercial license of MySQL. If you want to make sure that you don’t violate any software licenses, use the client DLL from MariaDB.