Frederique, this comparison of softness from Dakota Pastels might be handy for you in the future: https://www.dakotapastels.com/files/...tel-Lineup.pdf
The Koh-i-noor of course fall into the "hard" category. Dakota hasn't updated this with the Richeson soft hand-rolled pastels, but those are generally considered to be on par with Unisons for softness/texture.
I bought some half stick sets to start, but am now building my collection by buying open stock pastels. I have a whole big "wish list" and based on what I'm working on, different colors bubble to the top when I'm making a purchase, but it's fun to grow the collection. I found the half stick sets to be a great way to start, but you're right that there were inevitably colors that I am hard-pressed to use up from some of the sets (I may have to do some monochrome self-challenges to use those sticks ;-)
I have to say that a very valuable thing for me starting out was to buy a sampler set where I got one pastel each of about a dozen different brands. You may be able to do the same kind of thing for yourself from open-stock and for some colors, buy the COLORS you want, but across a few brands until you zero in on the brand (or 2 or 3) that you want to collect. While I think it was nice of the guy in shop to show you the two pastels and smudge them out, what you SEE and what YOU FEEL yourself working with a particular brand may be two very different things (as any one brand will also feel different on various kinds of supports/paper).
I love your portrait! And have fun starting your collection - everyone is right about how addictive soft pastels are :-)