As cappy208 notes, you are asking about two separate and distinct paths to mate of towing vessels. If you are considering pursuing mate 500 GRT inland, this may be the quickest path for you.
To go from Apprentice Mate to Mate of Towing Vessels, you will need an additional 12 months of expereince on a towing vessel after you are issued the Apprentice Mate officer endorsement (license). You would take an exam to get the Apprentice Mate license, but would not take a test for Mate.
Mate 500 GRT Inland requires a total of two years of experience. Assuming that you already have at least one year on vessels over 50 GRT, this will be the quickest path for you as you appear to be very close to having the two years experience. You would take a test for Mate 500, and once you have that license you will qualify for Mate of Towing Vessels after getting at least 30 days on a towing vessel and completing the inland TOAR.
Note that the above assumes you will apply for Mate 500 [U]Inland[/U]. If you want Mate 500 GRT Near Coastal or Oceans, it gets more complicated, more time consuming, and more expensive as you will also need to complete STCW training and assessment. If you intend to work near coastal, then going from Apprentice Mate might be better as it will avoid STCW (assuming an employer does not want you to hold a 500 GRT license and STCW).
Make sure that there is a designated examiner on whatever boat you will do yoyur TOAR on. It’s a fairly easy process to get approval as a designated examiner, so if you will work on a specific boat and/or with a specific captain, they can get themselves approved as DE before you start.