I have some of the Ikea devices that aren't supported by Hubitat and I'm in the middle of moving over from SmartThings, so my plan would be to just use the SmartThings -> Hubitat integration for those devices not supported by Hubitat.
So if you already have a SmartThings hub I'd say that would be the easiest way.
Assuming you do have the ST hub already I personally setup the official integration Hubitatâ„¢ SmartThings Integration - Why Hubitat Elevation - Hubitat
which was really easy to set up and get going.
You can't share buttons directly from ST to HE, however that's not a big deal as you can easily make some virtual switches in ST that are controlled by the IKEA buttons and then share those virtual switches with HE. That's what I'm planning to do myself at some point next week when I move a few more things over to HE from ST.
Note that the official ST to HE integration is only one way, which for me is enough and it's very simple to set up. If you need 2 way integration you can have a look at the community integrations for that.
As for a native Tradfri to Hubitat integration I wouldn't hold my breath for it unless you want to have a stab at converting the HA integration into a HE integration. If it works for HA I'd be pretty confident that it would work for HE, but I don't expect there are any developers that would have an interest in doing it
