It’s not just about how much storage you’re currently using, but also the rate that it’s increasing. I would look at the past few years and then come up with a “gigabytes per year” number for yourself, and then multiply that by 2 to account for growing resolutions.
Also: I hope that you’re keeping backups. Storing all of your data on your MacBook is risky because, if there’s an unrelated problem with your MacBook, you can’t exactly pull out the drive to get your data.
There are many factors that contribute to battery degradation. Cycles and age are factors (as you’ve already identified) but also heat and differences in manufacturing.
So, maybe you run more intensive apps than your girlfriend and, because of that, your battery gets warmer. Or maybe she just got a good battery from the factory.
This is why all of the situations that you’ve described are considered normal.