Learn something about your hearing first before buying or assuming custom is necessary.
Fit and seal affect LF response well into the hundreds of Hz, while ear canal shape and length affect HF above a few kHz.
As such, every IEM will introduce some level of immeasurable variability you'll have to learn to recognize. Almost all new IEMs with large casings sitting in the concha don't fit me, for example, and I get terrible bass response which doesn't match the measured curve. Most silicon tips are uncomfortable. My only comfortable and consistent pair are Etymotic ER4s with Comply foam tips. Good isolation, reasonable distortion, and some EQ for the bass and highs, and done.
"Generalist" or "music genre specific" IEMs don't exist. It's stupidity. Same goes for speakers and headphones.
"Transients", "detail", "soundstage" and almost everything else for IEMs is limited to FR since by inserting them in your canals you are removing almost all of the outer ear/head/torso cues you normally use to localize and perceive sound spatially.