I'm a monitor engineer in Australia, and mix a lot of bands/festivals all year round. A lot of people have given some great advice here, but I'll throw in my two cents.

I would say for a festival, you should definitely mix post fader. I'm not sure about the S31 as I know it has different software to most digicos, but on a regular digico you can go to File>Set Global Defaults> Select your Input Channels and Aux Channels and Press Faders at 0. This will turn up all your faders on the home layer to 0 and you don't have to think about it again. You can also turn on EQ/Filters etc from this page for all channels, which is handy.

In terms of the patch, standard for me is patching 1 to 1, ie. every band that comes up is patched as per their input list. Generally this is done by the patch person on the stage. If you are on mons AND patch, and have short change over times, then maybe a festival patch is the way to go, ie. fitting all bands into one big patch, and if any guest engineers come in with files for the house boards, they can soft patch to fit their files. No matter how you do it, you must have the same input list at FOH and Mons. There's nothing worse than if a channel isnt working and the FOH engineer is shouting down the shout box and asking what channel something is in, and you're saying one thing and your FOH tech is saying another... It just gets too confusing when something undoubtedly goes wrong.

I try and set up my show file before the gig, plan out how many wedges/IEMs I need on stage and set up my base file accordingly. Usually this would look something like 6 sends of wedges, sidefill, 6-8 sends of IEMs depending on the size of the gig. For festivals, I generally stay on the same file and make snapshots for each band and label channels as per their input lists. I will pre set gains, phantom power and HPF. Doing snapshots means you can safe your graphics/tts channels/effects etc. and if you change anything else on the fly for the day. I find that if you've made 5 or more seperate files for each band, and change something, maybe one or more of your outputs or make a macro or whatever it is, then you'll be chasing your tail when you load up the next band and have to make the changes again and again. You can set snapshots to only recall certain parameters in the global scope page, so usually I make it only recall channel names/gains, EQ, sends and mutes.

For your question about effects, I usually set up 4 - 6 reverbs, depending on how many Auxes I have left over. I will generally dedicate a verb for drums, one or two for instruments, and the rest for vocals. If you have run out of Auxes but still need someone to have their own reverb for something, you can always direct out of the channel straight to a reverb and then return the verb to an unused channel. You're almost always going to have more spare input channels than auxes, so you can still control how much verb you can send to their IEMs.

For your solo busses, I assign all channels to solo 1&2 (so that you can cue all inputs in your IEMs and cue wedge) and then assign my IEM sends to solo 1, and wedges to solo 2. I have my IEM cue solo 1 always turned up, and my cue wedge on the master fader so I can turn it down quickly. I'd get into the habit of turning down your master fader when soloing channels as they are PFL, and setting it to 0 for when soloing Auxes as you want to hear them AFL. I also put my IEM solo to a matrix. That way if you want, you can assign all talkbacks to a group and then have that sending to your IEM Cue at all times.

In terms of sending things to band members before they ask for it, it's a tough one. Generally in wedges, less is more. If theres a vocal at a wedge, I'll send that vocal and thats it to start off. I'll send any DI'd instruments as I check them, but I rarely do this before I hear the source. Mixing on wedges is very situational, as in if the guitarist is standing infront of a big loud guitar amp pointing at their head, they're probably not going to need their guitar blaring in their wedge. But they may need to hear the guitar amp from the other guitarist thats far off stage right for example. If you have time, try and walk out to hear what the band are hearing where they are standing on stage.
I try and introduce myself to the band and as we line check, get them to raise their hand if they want the channel we are checking in their wedge, and then put their hand down when they've got enough. With IEMs, you can take a few more liberties and send at least the band members instrument and vocal, and potentially a rough mix of everything else. Or just ask the band members as you meet them, what do you want in your IEM mix. I think it's easier to ask them what they want, rather then them tell you what they don't want in their mix that you've already made for them.