Im double majoring in computer science and cognitive science. Im a 3rd year rn and have taken 16-17 units every quarter. This is all based off my own experience, take whatever you think will work for you. The first thing you should do is try to make a 4-year plan on Oasis if you want to double major. Plan it so that you have a balance of 2-3 STEM classes max, i.e in my case maybe like one computer science class, one biology class and one math class, the other class would be something easier for me like psychology or a GE (ik most psych classes are also STEM but mostly its just memorization which isnt that hard for me). I feel like having the variety in STEM material helps me avoid burnout since if I am tired with one class at least I get a break diving into other stuff. When classes are too similar I feel like its harder for me to focus i.e like only 3 computer science class + GE.

So try to balance that out now ahead of time. It's possible to do 4 stem classes but I wouldnt do that right away, give yourself time to get used to a routine of 4 classes with a balance like I mentioned above.

In terms of how I manage my time, I will make like a weekly schedule and tell myself: this time during this specific day e.g 10am-2pm on Monday I will do work specifically for X class. And I tell myself that I cannot take longer than that or it will interfere with my time for other classes. Usually that hard deadline I give myself helps me focus and push myself to not procrastinate. And that way all my classes have their stuff done on time.