My experience, been gigging since the 70's, is:
1. If the sound people are good, you don't have to tell them how to do their job. Just act professional and let them do their thing.
2. If the sound people are bad, no amount of hassling, explanation, or trying to teach then about audio at the show is going to make much difference once the show starts. And if you get all frustrated with them, they probably won't give you even their best effort during the show.
For festivals where you are not the headliner, there are strict time constraints for loading in, playing and loading out. I try to be prepared. Know my rig inside and out and be able to setup, tear down and diagnose and fix audio issues fast. I also like to keep the rig as simple as I can; especially if it's only one set.
I bring my own powered wedge and small mixer to mix from the stage. I recently upgraded from a small format Mackie to a Rane rackmount mixer thanks to a post by
@hippietim on TPG I think? anyway really cool piece of kit that will let me mix my vocal, my guitar, the lead vocalist's guitar and still have one more channel for an aux mix if I need it; all with a decent 3-band EQ per channel. Will be playing a big local festival soon and testing it out.... no dependency on the festival monitor mix!
I go for a balanced tone and don't try to cut too much low end. I favor the mellow side of the treble as I find really knarly spiky presets to get fatiguing playing a long show with FRFR.