Is this bulb capable of being paired and controlled directly with HE? Does this bulb repeat? That would also solve an issue (Icing on cake). Do you have to also have the Tradfri remote, gateway, etc?
Anyone using these for similar use case? What has your experience been?
That bulb is on the compatibility list and should work. I have what I'd say is the same one except the Ikea part number looks different, so I wonder if they did a hardware upgrade or something. That being said, there are warnings about most directly-paired Zigbee bulbs in the docs, worth reading. I haven't seen anyone call out these in particular, but I'm not sure anyone has sat there with a Zigbee sniffer for a while to objectively see.
The Ikea bulbs are, in my experience, a bit slow to respond. This isn't a Hubitat thing; the same thing happens on their own Gateway and when paired via a Hue Bridge network (which is how I'm using mine now). It's not as noticeable with these but is pretty bad on the color bulbs, which will not work well with Hubitat for other reasons (they implemented only one of three possible Zigbee color control/reporting methods, basically, and Hubitat uses only the other two--true regardless of whether they're directly paired or used via a Hue Bridge).
To answer your other questions: these bulbs do repeat (may or may not be problematic, as above). You don't need the Gateway or any Ikea device to use it, and in fact there is no Gateway integration, so you can't use that, and most Ikea accessories won't work on Hubitat either (some could be "Touch Linked" directly to the bulb if you really wanted). You could still use the Hue Bridge without Hue bulbs (these work there) if you wanted to. You also can't directly control Zigbee routes, so there's no guarantee that a single repeater will solve whatever problem you're having (assuming it's not a problematic repeater in the first place), but hopefully it helps!
I have one of these and it did repeat when I had it directly paired to HE.
I'm not actually a big fan of this blub because of only having 3 temperatures you can set so if you plan on using that maybe look elsewhere.
Also based on this thread your mileage may vary with the tradfri bulbs.
