Hi,
Choosing your career is always upto you, when you went to pursue masters then there should some mind set that if you come down with the degree then you would get paid more or its your wish that you want to pursue this in your future.
Having your experience in full stack development for two years, its a added value to your career, but i would suggest you to pursue the role you are presently working in because many companies are there which actually pay well in Designing domain, and if you are having a degree certificate on this it will take your career in a different level.