Prioritize specific firewall zone throughput (to WAN) or rate limit other zones

I have two firewall zones of interest lan and rent, both of which can forward to wan. My goal is for OpenWrt to preferentially forward traffic from lan to wan when there's too much traffic competing to forward; if there's not a lot of traffic from lan, rent may take full advantage of the bandwidth. If such a solution is not possible, I'm willing to compromise to rate limit rent.

As per my google-fu, nft-qos appear to be what I need to use. But oddly enough, there's no documentation to be found on the OW wiki.

I installed the luci-app-nft-qos package, and the Limit Rate by IP Address tab seems straightforward enough. Though it won't let me select specific interfaces or firewall zone, I can copy the netmasks corresponding to rent easily enough. But that's for rate limiting, my last resort.

What I'm most interested in is the Traffic Priority tab. It lets me Enable Traffic Priority and lets me choose Default Network Interface. Now, I'd assume I should choose the interface associated with the lan zone here, and it'd do exactly what I'd set out to do: preferentially forward lan traffic. But without the documentation I'm actually quite confused what this is supposed to do. The interface drop-down lets me select the WAN interface as well; what does this mean? What will happen to the interface's traffic, whether it's the WAN interface or one of the others that are defined? What if I want to assign priorities of each interfaces in order?

Or is there some other solution entirely that I must be looking into?

Traffic Priority is not what I expected it to be. Only after I enabled it, I realized it simply prioritizes on traffic on defined ports of a specific interface. At this moment, there doesn't seem to be any way to do prioritizing specific network segments, only rate limiting.