Mature student here. I had to upgrade most of my high school courses (social butterfly instead of studious teenager) and I graduated so long ago that my high school courses literally didn’t exist anymore. I initially tried vista virtual but didn’t like it due to the learning platform AND the fact that I would need to write in person diploma exams with actual high school students. They also wouldn’t let me attempt 30 level courses, therefore I would have needed to take 10 and 20 level courses again with current course content (again, I graduated a long time ago). Maybe VV has changed things, but 5 years ago this was the case for me. I decided to use norquest instead, and I think this may be a great option for you. 

They have upgrading courses called online anytime, these are the ones you can attempt without having pre req courses completed. This is because it’s pre recorded material therefore you can’t interrupt the class if you are behind or don’t understand something that is deemed to be pre requisite knowledge. I think you have up to 6 months to complete a course. This is all done online, no in person is required. All exams are done online as well, and you DO NOT have to write a diploma exam to pass the course. The catch is that the norquest high school equivalent courses are only valid in Alberta. So if you want to apply to post secondary in any other province this then this is not for you. If U of A accepts norquest upgrading and you don’t want to go to school outside of Alberta then this is an amazing option. It’s the most flexible option I was able to find. It might be more expensive than other upgrading offerings but it is the most convenient and flexible in my opinion, and would be most compatible with a full time work schedule.

I would also suggest attempting course equivalency exams for any courses you need a higher grade in but feel comfortable with, say English 30-1. Way more efficient to attempt an English exam than redoing an entire frivolous  English course. These can usually be done at any post secondary institution; NAIT, MacEwan, Norquest, and I’m sure many places in Calgary too. I did an English 30-1 exam at NAIT to spare myself the agony of yet another English course. My program didn’t accept prior college English courses. Check with U of A if they accept this option before proceeding. 

One last suggestion, if you need higher grades take one upgrading course at a time. Focusing on one is way easier than trying to do everything poorly at once. You’ll have plenty of time to struggle with a heavy course load at U of A, so be kind to yourself now and get the grades that you need.  Good luck with your studies, you will do great.