Happy to help. Shoulder position and patience off the floor are the biggest elements of sumo DLs for me.
Enjoy your first meet and best of luck. Relax and have fun, don't worry too much about the weight on the bar. I had my first two meets not three months ago. I wouldn't worry about PRs until your third attempt: I went for 90% for my first, my training max for my second and then a 10 lb PR for my third for my first meet.
Chucks are a great idea. I got some Matflex 4s for sumo, and they're all I wear now, except for squats. I tried pulling sumo for my first attempt at my last meet in my Dunlop Volleys and ended up rolling my left foot out of the shoe and onto my ankle. I just about locked out the 450 lb on the bar but fell on my backside. Went back to conventional for second and third.
Why the high reps? I've found one to five works best for me; well, one to three for DL, one to five for the others. I don't often go over five and if I do it generally won't be above 70% or so.
One thing I've found that really helped my sumo even from small amounts of volume are high box squats set so the hips are at the same height on the box as when the pull starts. I load that up with 80% of my max DL or so and work upwards for triples. Something about it really helped me get my glutes moving the weight from a dead stop. For deficits I prefer pulling snatch grip, it just feels better.