Sound is too personal; but if you're starting out, and if possible, do a test-run with different sound signature before committing to a custom. It's a big investment. Do you have an audio store nearby that you can test in person?
If trying out in person is impossible (understandable in these times), you can also get a bunch of cheap Chinese iem with different sound signature (like under $50 cheap) and see. My guess is once you find one that suits your taste, you can expect (normally) a better refinement on such sound going up the price. Of course this is not always the case, but you get the idea.
Crinnacle and others have graphs that can aid you in identifying the sound that you like once you have some experience. From your list, if you like the Buds+ (i have them too, they're nice), you might enjoy a bass boosted or V-shape sound or Harman. Again, this is up to your ears and taste so my advices can only do so much. It's a fun and costly journey (sorry about your wallet).
Regarding usage with Fiio UTWS3, it's the 2nd step after your journey of obsessing over different iems.
The custom mold isolation and comfort depend greatly on your ears. Some people swear by it, some don't (like me). Just whatever you like really.
do a review
Also, I was wondering if anyone can measure the changes in sound between wired and UTWS3. It sounds excellent alright, but seeing the actual effect would be interesting
The Jabra and Buds+ are V-shaped or harman tuned iirc. Dunno about the Klipsch. But this is the common consumer tuning which is 'bass' heavy and somewhat brighter than neutral.
The Tin T2+ is very close to DF neutral (minimal bass and a 'pinna' gain). The Openhearts are V-shaped or bass heavy iirc. So thats 2 of 3. For bright or treble heavy I'm not sure where to go, the original Tin T2 would be one but fit doesn't work well with over ear iirc.
That all makes sense; I'll look into this topic more. One last question (trying not to derail the thread too much): do you have any recommendations for finding a good audio store where I can test in person? I'm currently near Grand Rapids, MI; I did a quick Google search of this website and didn't see any discussions of audio stores in my area. In any case, you've both definitely given me lots to look into, and I really appreciate all of your help!