300 hours is a benchmark that does not fit everyone. In my case, for example, I didn’t count the hours I studied, I counted the exercises I solved, which is a more tangible measure of the performance evolution you have developed. If you want to know my opinion, knowing you have passed both FRM exams you will need lesser hours, however, the exam style could be different, so practice a lot of exercises.
I know that FRM exam is 4 hours straight in the morning and 4 hours for the part II but it is optional to take it in the same day, so I think you are used to do 4-hour exams only. CFA exams are 3 + 3 hours in AM and PM sessions, so this is a point to consider. Practice mocks.
The questions give 3 options only, not 4 like the FRM exams. I think this is an advantage for you.
The key word is “practice”. Good luck in this new journey of yours!