If I understand correctly - it’s not your 200 PR that is 21.9, but your OPENING 200 in the 400?? Quite a difference!
You can do a RESEARCH of the Lactate Threshold thread - you will definitely get AWESOME hints and knowledge there.
From my point of view - I have two 400m athletes (male), totally different regarding strengths and weaknesses. The one, running 21.5 for his first 200m, PR of 45.3 in the 400. (More speed orientated athlete.) The other - 22.0 for his first 200, 46.3 in the 400. (Endurance stronger than speed.) (Both will improve tremendously in the next season - based on their base training now!) BUT - I won’t change their first 200’s! I am working on the second 200’s - if that can improve, both will be able to improve their PR’s.
Based on this - I would recommend that you work on your speed, speed endurance and special endurance, but be careful NOT to try to run the first 200 TOO fast at this stage. Work on the LAST 200 firstly in your training. (I have learned this from Kitkat!!) When the difference between your first and last 200 is closer to 1.5", you can attempt a faster first 200.
Good luck with the training!