It's always worth doing a conversion course for whatever products you are going to be using.
If you're not sure what you want to do, your best bet is to ring NSI and speak to them.
To be honest essential nails is a starting point but there is so much to learn any education is an advance for your career.
I'm not sure why you feel you don't need to do the conversion course. All liquid and powder is different. They all have different properties and ways of working it. It's better for your business/skills to know the difference and how to get the best out of your chosen brand.
If you're after a "master" qualification then can I ask how long you have been qualified? Would it not be more pertinent to walk before you run? Master the basics and learn your craft first. Qualifications do not make a first class nail tech, they merely equip you with the knowledge, skill and understanding to make start. Its how you apply that knowledge and skill, and practice, practice, practice. Xx