Yes, all the themes on wordpress.org are free. There's not a single simple answer about those themes being "better" than commercial themes -- as so much depends on what you need and want. But all the themes on WP.org have been tested and meet WP standards -- which is a good thing and a reason to avoid getting themes from shady vendors or iffy places. (There are certainly many good commercial themes out there too -- and many of those vendors are also listed on WP.org -- as recommended commercial theme sources.)
I certainly understand and agree that it would be hard to give up now on all the work you've done -- and I'm sure you'll be able to figure out the rest of what you want to do.
But one of the very cool things about WP is the great flexibility and so many choices you have -- so you can try things out, experiment, etc. (Not suggesting you do that with your current site, but just that it's out there for "next time" or when you have time to fool around and play with some things...) Download a few themes, create some content and see what you can do.
I also totally agree with learning more is always good and helpful and interesting and fun (so long as you are not banging your head on the desk -- LOL!). There are lots of tutorials and info about WP (and PHP per se) out there on the web -- including YouTube -- so you can no doubt learn as much as you want...)
And yes, it's always helpful to mark threads resolved -- it helps keep things tidier!