This thread has most of what you need I think.
Note that the style of custom IEMs I use have the drivers etc integrated into the mould itself. I think you can get custom moulds for the ‘ear tips’ of some other brands, there’s an example of a Shure like that in this thread. Others will know more about that. The rigid ‘hearing aid’ moulds may look uncomfortable but they aren’t for me even on extended listening. There’s no sound leakage and the good fit means great performance too. I have two different JH Audio pairs now. The better of these is up with my best headphones, although the IEM experience is different of course.
If you look at the Custom IEM Co website (sorry I cannot post a link) you will see some models are definitely aimed at musicians for live performance etc but others have more of an audiophile profile. They are extremely helpful on the phone and via email, and can arrange demos and also moulds. My local audiology clinic made my moulds.
Let me know if you still have queries after digesting this lot!
Bruce