Studying over the summer could work out for you. But only studying the topics will not be enough. The difficulty of the questions of the IB Math HL exams comes from the structures of the questions. You might think you know very well a topic, but you might find out that it is hard to solve a problem on that topic because the question usually will not be a straight forward one. In order to solve it, you will have to use your knowledge of different topics.
So, it is doable only if you study hard, if you try to really understand the topics (and not just to memorize some trics) and if you solve lots of questions on each topic (therefore if you solve all the questions in the past IB exams). One of my kids (who was not strong in math) dit it, but she had to allocate half of her study time to math!
If you want to take a CS course, you can take an online AP CS course. You can find it from many institutions in the US (JHU CTY is one, but there are others). When I say online, it will not be a course that you study alone on your own pace, but a course in a virtual classroom with students from all over the world (and with a teacher of course). If your school accepts it as one of your six courses it might be more doable, otherwise it wouldn’t worth it in your IB years (you won’t have enough time to study and to do the homeworks of a 7th subject).
One other strategy for you might be to enroll to 4 HL classes including the Math HL and then drop one of the HLs to SL in the middle of your junior year (of course if your school allows such things). That way, you could see how hard is Math HL and then you could decide on whether or not to continue with it.