Apple do not provide any QoS or indeed just about any other ability beyond the most basic.
Setting limits to usage is not easy though. There are extremely few routers out of the box that can do it. None in the domestic market.
You can buy a router.. not expensive. eg Netgear WNDR3800 (hard to get new now, but easy enough on ebay)
Or TP-Link WDR3600, and flash it over to gargoyle firmware. I am not allowed to give you URL (apple delete my posts) but you can look it up easily enough. Flash is easy to do.. this is nothing more than a firmware update but using a non-standard firmware. The firmware includes throttling and quota based on IP address. So the apple routers can all work in bridge to it.. you can provide guest via the Gargoyle router or use a apple router plugged in by ethernet as guest.. and all IP on that guest account will be throttled. (Guest is not possible on the apple router in bridge, so you will use main wireless with a different name, since it seems you have several units).