Thanks for the follow-up, and I apologize for my own slowness as well.
I have actually figured that to be the case, i.e. needing the OpenWrt device to run as the router and not just as an AP. Because after I downloaded the nft-qos package, and set some bandwidth limits, I didn't really see an obvious effect on the devices I tried to rate-limit.
I haven't gotten around to doing this yet, but my next plan is to connect a separate router (already have this device ready) to my OpenWrt AP, which is a TP-Link Archer C7. This other router will serve as the gateway for a new guest network. I have flashed FreshTomato onto it, which comes with a built-in QoS management system. Devices I plan on limiting will only have access to the AP broadcasted by this new guest WLAN router, whose outbound rate I plan to limit to something like 30-40% of that of my ISP's.
Hopefully this will be able to get me close enough to how I want my network to function, which is to allot guests (we host actual long and short term guests through Airbnb) a certain amount of bandwidth on my network, but not exceed the maximum limit I set. So that myself and the rest of my family will always be guaranteed a minimum level of performance even if some guests are using the network to an excessive degree, which though not too often, does still happen from time to time.
Do you think this is a good plan for achieving what I'm looking for?