I'm also having an issue switching inputs on the 600 via the Harmony Ultimate. I have tried telling the Harmony the projector was a 500 and a 1000 because the 600 isn't in the database. That doesn't make any difference, 500 acts the same as 1000 for me.
My Harmony's INPUT soft button works fine. I can manually change inputs with no problem.
The problem is going to the "Watch 4K Content" activity, which is supposed to switch the 600 to HDMI 2. It stays stuck on HDMI 1. The projector blinks, its input menu briefly appears, but does not go to HDMI 2. I can manually go to HDMI 2 easily and use the activity. (The 4K server is always on, so there is always signal going to HDMI 2.)
There is no problem going from the Watch 4K activity to any of the activities which use HDMI 1. The projector happily switches back to HDMI 1 from HDMI 2 as part of an activity change.
Summary: the projector won't switch from HDMI 1 to HDMI 2 as part of an activity change. I have played with all the Harmony delays without any luck, but because it will switch from HDMI 2 to HDMI 1, I don't think it's a Harmony delay issue. If anyone has a solution to this particular problem, I'd love to hear it.
So discrete commands would be nice. End users can't add them. You have to contact Logitech and ask very nicely pretty please add these commands.