Thank you for the follow up.
I only have 3 VLANs and an untagged LAN (a protected LAN, an untrusted IOT, a higher-reputation IOT, and a guest VLAN - seldom used). Accordinly, I have 4 SSIDs. The 3 VLANs are wireless only, and they are firewalled at the router level. There is no broadcast leak between the VLANs (at least not at the router level) but I do see the broadcasts at the wired level as expected and as you pointed out.
I understand that Proxy ARP at the AP level would cause the AP to reply to the wired router instead of forwarding the ARP request wirelessly to all (or each) subscribers (either as a broadcast or unicast), thus reducing radio utilization. That was the original question. Multicast to unicast conversion is certainly helpful (thank you for the confirmation).
My remaining question then is: how is the radio utilization calculated? Is it radio time, number of suscribers, other formula?