That's a good question and I asked me this a lot. I'm new in the audiophile world and I started with buying the Focal Stellia as a closed headphone. I think the most interesting thing beside enjoying my music in really good quality is an "excitement" (wow) factor. The Stellia has this factor, because of its technical capabilities and his bass quality (most deep and hard rumble/punch of all closed headphones I've tested). Than I bought the Focal Clear OG, too. This headphones plays everything really really nice. It's really a headphone for enjoying your music, but has not that much of an excitement factor beside that it plays music so well. Than i bought the Hifiman XS as allrounder (the Clear is an allrounder too), but the comfort is much better for me and sounds more open. So it's a little bit my "always on" headphone. The Arya was the first headphone which had in comparision to my headphones a stellar technical factor to all of my headphones: The soundstage and immersion was fantastic. The difference to the XS is not that much, but this extra few percentage is the extra that makes you really excited about the music listening. I had a hard time returning the headphones but the comfort wasn't the best for my large head and overall it was too bright for me. The XS has not the technical capabilities, but is exactly my preferred tuning.
So to your questions with the IEMs: I thought the IEM's can't give me much excitement, but good sound quality. I tried as first kilo buck IEM the 64Audio U12T and this IEM is really good, but for my taste only technical good. The Focal Clear OG is much more enjoyable for me, because of the DD Bass and has much more flavor. I could understand why it is an expensive headphone. After the U12T I tried the 64 Audio Trio and this IEM is really different. It does really things that makes me excited about Music listening and I was fascinated that this came out of the little IEM. It's for sure not comparable with the Arya in soundstage or immersion. But the Trio is it absolutely worth. I think the price/performance ratio is the best with open-back headphones like the Focal Clear OG. I think with open-back headphones you have the most immersive music listening, but I think that IEM's are at least interesting as closed backs and the form factor is so nice. It's really nice that you are mobile with headphones like the Trio and have that good sound.
I tried the the Sennheiser IE600, Shoer S12. Both are really good for there price range, but for me they do nothing spectacular. So no reason for buying them. I think the U12T is not worth over the Clear (U12T is probably more technical, but Clear has the better Bass), but the Trio is worth over the Clear. I think about to buy the U12T for the Mids, because the disadvantage of the Trio is that the vocals and mids are a bit recessed.