Setting up wifi for a local festival with an estimated 5000 (potential) devices, someone hold me I'm scared

I feel very out of my depth but here goes:

the company that I work for is providing wifi for a local festival and I've been tasked with making sure it goes well.

I'm planning on using a netgate fw (or similar with QoS/traffic shaping) and ubiquiti high density APs for providing the setup. of concern is that they are going to have lots of those ipads with square readers on them for payments and I need to make sure those have priority to bandwidth.

I am setting up two VLANs, one for Guest and one for POS systems.

I literally learned about

https://en.wikipedia.org/wiki/IEEE_P802.1p

while setting up a test bed just now, and I'm wondering if I add in QoS/traffic shaping on top of this if it will help, do nothing, or hurt performance of the network.

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also i suppose while I'm here if anyone has setup stuff like this before please drop your protips. In my head I'm setting up a portable rack with my firewall and a POE switch in it, then just connecting all my APs up to the switch and setting it up like it's a normal office network, but I've gotta be missing details here.