If you just want to keep old skills sharp books like this might be helpful. For learning specific new topics I'd start digging around on math.stackexchange, syllabi at local universities or math related subreddits for textbook recs on the particular topic.
I have not used Brilliant, so I cannot speak one way or another for their course. However, I am a mathematics undergraduate who has worked with high school and undergraduate students for approximately six years in mathematics and foreign languages. (I am currently employed working with students learning English from a non-native English background.)
In my experience, practice problems are king. Students can watch lectures, full or abridged, all day long and, for most, it just won't "click" until they have tried a handful of practice problems of various difficulty.
I always recommend Khan Academy because I have used them myself and I have colleagues, friends, and students who also use them. We have all had good results. They are a completely free, non-profit resource dedicated to educating the world and I genuinely admire their mission and effort.
Because you're getting ready for calculus, I would recommend Schaum's 3,000 Solved Problems in Calculus. If I were to go through the calculus sequence again, I would want to gift that book to myself a semester before I began my first day in calculus. (Actually, I have this book sitting on my desk next to me now as I type; I like to do problems in my "unwind" time.) The first five chapters cover things you will need for calculus, and then the rest covers topics from all of the calculus sequence: calc 1, calc 2, calc 3, and differential equations. Each problem type has multiple practice problems of varying degree and the solution and a short explanation lies directly below each problem.
As you prepare for calculus, I would like to impress upon you that calculus is not a difficult subject and that, in my experience, not one student ever fails calculus because of the calculus, but because of the algebra. If you have very strong algebra skills, there is no calculus problem you will not be able to solve.
And, finally, I would be remiss if I did not insist upon your asking any question you need at any time right here on r/learnmath. We look forward to your questions and are delighted to help you!
Schaum's 3,000 Solved Problems in Calculus is the one book I would gift to myself if I had to go through the calculus sequence again. If I ever work as a calculus professor, it will be on the required books list for my course.
Khan Academy, Paul's Online Math Notes, PatrickJMT, Professor Leonard, r/learnmath, and the plethora of other resources out there will see you through understanding the calculus sequence no problem.
Is your objective to build a comprehensive understanding of the underlying topics of Calculus or is your objective to master quick problem solving, tricks, etc? If it is the latter I would suggest you pick this up as an auxiliary resource; Stuart is good but mastery of the mechanics of solving the problems will come only through ardent practice. You will need to see, and solve, a wider set of examples than is typically found in Stewart.
If your objective is the former I would grab this instead. Probably look for it on a used book seller's site like abebooks.com, though.
I'm going to go with a slightly different approach than starting from the very beginning.
How much do you know about calculus? If you know the basics of limits and derivatives, I would suggest to start learning at calculus. Go along with what you're being taught in class.
You can use Khanacademy/PatrickJMT to help you understand the concepts being taught in class. At the same time, as you're going through each concept, look up every term you're not familiar with. Don't take anything for granted. For instance, if you come across inverse functions in the explanation of something else, can you explain what inverse functions are? What's the difference between inverse and reciprocal? Or for the unit circle, do you know how the values came about? Question your understanding on every one of those concepts, and Google every single one of them. As you're going through the concept, make sure you commit it to memory. Try to build on your understanding. Even if you forget a little bit, the next time you come across the same concept, you'll have solidified your understanding a little more. The important thing is to be conscious of what you've just learned.
With this approach, it's going to take much more time and effort than your peers to get through some concepts, because you're using the opportunities in between to touch on previous concepts as well. So you really have to budget your time properly, but it'll be worth it in the end. If you don't have too much time, don't spend too much time rolling off the tangent looking up every single concept, just look up the thing that comes up and commit that to your memory.
Because you're going through a course, you don't have the luxury of being able to re-learn every single thing since grade one. The approach of learning as it comes up is much better suited for this situation imo. It's scary thinking there's a lot of things you don't know, but you can tackle those concepts as they come along. Don't panic.
Then at every available opportunity (winter break for example), practise what you've learned and drill yourself on the concepts.
I had a very similar problem of feeling like there are holes in my understanding and this was the approach I took. I'm in the middle of Calc 2 right now. As we're heading into winter break, I'm going to be reviewing everything that was taught this semester in Calc 2 and to review integration to prep for the second half of the course. I'll also be drilling myself with Shaum's 3000 problems book.
There are some good suggestions in this thread on Math Overflow as well.
Good luck!