Assuming you mean the Beelink S12 (which is the first thing that comes up in a search for "n100 mini pc"), that's quite similar to my own computer specs, which can run just about any distro, with enough resources to spare for a VM or two. I don't think it's necessary to go really lightweight or pick something special. If there's a distro you're already familiar with and know you can do all of those things on, install that.
If you like Garuda, you could always try a different Arch spin which is lighter out of the box, like CachyOS or EndeavourOS.
I'm currently on the RHCSA path myself, and I can tell you that the courses are not worth the thousands that Red Hat charges. There are plenty of unofficial video courses on YouTube and Udemy and study guides and practice tests on GitHub that are free or cheap, and other resources for every individual study topic, which will be good enough.
However, though I can't speak from experience, it seems like the cert itself will look good enough on a resume to justify the investment of $500 and a month of studying.