Should I go with HTML5+CSS3 or CMS for a 7 page business website? Why?
A WCMS will improve your time (reduce cost of) updating pages. Generally, the smaller the number of pages, the less overall updating you do, but that's just a rule of thumb. The key here is to ask how often you update these pages, how long you typically spend when you do. Then compare that to the time and cost of deploying a WCMS first, then the cost of learning and using the WCMS vs native html+css skills you appear to already have.
Now, another factor is that whoever is hosting your actual live web site probably also provides some basic WCMS for free. If that is the case, then adjust the second part to simply the time/cost of learning the WCMS that is there vs. just uploading complete html+css files to a static site.
Finally, there is a whole precursor analysis you may want to do. Namely, how often *should* you update this site? Sites that are dynamic and fresh with content have much higher traffic, inbound lead generation, and other business benefits. Often the decision to deploy a WCMS is made in order to force yourself to have a constantly fresh site with lots of updates.