Shure se535 upgrade recommendations

Use: monitors at gigs for guitar player

Budget: <$1500

Goal: flatter frequency response than 535s and custom molds

I have a 12+ year old pair of 535s that I use live almost every weekend. I know IEM technology has improved and that there are better performing monitors now. I'd like to upgrade to new monitors with custom molds. But I can't really figure out how to choose a manufacturer and model. I'm looking for a fairly flat frequency response with more extension than the 535s offer which are very mid focused.

I just don't want to spend the money on drivers and molds just to be disappointed.