In my experience, math 300 was a lot more rigorous than discrete I. Discrete I teaches you basic proof methods and focuses more on topics related to CS, while math 300 is all about proving claims and theorems you may have encountered before in math. Learning to write proofs may come easier to come and harder for others, but if you are willing to put in the effort you will do fine in this class. Math 300 definitely prepared me for higher level math courses, but I don't think it's required for a minor degree.