I was looking at uWaterloo's CS Degree with a Bioinformatics option and it seems roughly equivalent to a CS degree with a Bio minor. What are your bio marks so far? Have you taken any CS courses, any good marks? You don't want to do med school anymore but do you still like biology? If not you should just go pure CS if Biology is gonna bring down your GPA. I feel like a double major might be a bit too much unless you want to work in a wet lab type of setting for biology.
I think if you're interested in both Bio and CS you should aim for something in the bioinformatics area. Possibly consider transferring universities? Here's a link to
Undergraduate Bioinformatics programs
The closest you can get to Bioinformatics at McMaster for undergrad is Bio 3S03 but even then, 3S03 requires Bio 2C03 (Genetics, boring af and I dont know if you still like Bio) and 3S03 is a disastrous course in my opinion. It covers material from Biology, Math, and Computer Science. The Biology concepts don't need to be understood, just memorized. The depth of the biology material covered is pretty shallow and I think someone who hasn't even taken high school biology could probably understand the concepts. There's lots of Math and Computer Science concepts introduced at an insufficient depth but most of the students lack the math background to even understand the concepts anyways. But you're never really tested on any mathematical or CS concepts so I really don't understand why it's introduced in the first place; it's relevant to the field but not to the course. It's kind of a "Jack of all trades, master of none" course because you really do not need to understand anything as long as you can memorize the material, which in my opinion makes no sense for a bioinformatics course.
There's also Biochem 3BP3 which I've heard actually has coding assignments? I assume this course can't be worse than Bio 3S03. I haven't taken Biochem 3BP3 so you'd probably be best finding someone who has and can give you the details. However, since it's under the Biochem department it won't count towards your bio minor and will need to be taken as an elective.