This is just my opinion but I’m not a fan of IEM’s.
Now, admittedly I haven’t tried the high end ones. But I will tell you this.
If you buy a set like the low end basic Shure 215 set you’re going to hate them. They are a single driver set and while vocals sound great your guitar tone that you have worked really hard at to sound pro, will sound like you’re playing inside of a tin can.
For guitar/bass/drums you’ll need a set of IEM’s with more drivers. Now, more doesn’t always equal better. It all depends on the application and what you want to hear in your mix. For guitar you’re probably looking at a quad driver set or minimum 3 drivers from a quality brand....and that’s going to cost a pretty penny.
As far as a sound man wanting to turn you down, I’d suggest looking into a small plexiglass sheild. You can get them at guitar center for about $100. This way you can turn up enough to get your tone and because the amp is essentially blocked you aren’t ripping people’s heads off with volume.
Then you can turn up your mix in your IEM’s or floor wedge monitors.
Spend some time looking through this website...you’ll learn a lot.
https://alclair.com/driver-ed-driver/