Maybe someone more versed in UniFi will step in but from what I've experienced throttling in UniFi exists across the entire network, not just WAN egress traffic. It severely limits it's usefulness.
While this isn't exactly "per device" throttling it looks like it can be used to achieve the same thing. I would most likely have a "low speed" tier that I assign to things like smart speakers, kids devices etc.
I swear I saw some screenshots or maybe videos with a per-device throttle option but maybe I'm getting my routers confused.
