What region are you in?

You could try some Motorola DTR or DRL radios if you want something license free and not super expensive. Motorola claims they cover 300,000 sq ft indoors. They put out 1W on 915MHz, they use frequency hopping spread spectrum, and they have digital audio.

How's your WiFi infrastructure? Assuming you have the area covered with WiFi you could go PoC (PushToTalk Over Cellular). There are dedicated handsets you can use for this for as low as USD$50 ea on Ali Express. Being that it's just WiFi there are no licenses to worry about. Having a 2 way radio type handset makes it pretty familiar, buy you can very much use it with existing mobile handsets. Zello seems to be the de-facto standard for this but i believe other options exist.

The problem you're going to encounter is that any radio system capable of covering your entire area is going to need some sort of license, either individual or group. You probably can't require your volunteers to pay the FCC $35 for a GMRS license, and ham radio requires an exam, so that's probably out, too.

That pretty much only leaves business licenses. Yes, there is MURS, which is license free, and might work better than what I am assuming are Walmart FRS radios, but still runs the risk of communication failing when you need it.

If I was in charge of a convention, I'd talk to the company that supplies your AV equipment about who to rent business radios from. They probably have recommendations. If you don't have an AV company, you can Google for "radio rentals West Virginia", there's a few options. They can help you with all the licensing, and will provide all the radio hardware. This is not going to be cheap, but will probably end up being less expensive than trying to set up your own repeater, and all the associated licensing headaches.