Hi All
I’ve got the Harmony Hub paired with an iPad and iPhone to control Roon over Bluetooth.
I added a Harmony Pro / BT Apple TV 4 device and then created an activity with all my components I wanted to control.
Then I added a keyboard device to the BT Apple TV4 and paired Harmony with the iphone/iPad.
I was able to then customise the Harmony Elite buttons for the activity and edit each button to a corresponding BT Apple TV4 command. (Home, Next, Back, Play, Pause, Stop, Left, Right, Up, Down)
There are a few other commands available but they are the ones I wanted.
Works perfectly no lag and every time to control playback on the iPhone/iPad from the Harmony remote.
The iPhone is connected to my iFi Micro iDSD Signature so now it’s a completely BT controlled and Roon Remote controlled streaming DAC. 
iPhone BT Harmony Pairing by Hilton, on Flickr
20210310_212152 by Hilton, on Flickr
20210328_184033 by Hilton, on Flickr
Using Nvidia Shield as Roon Display device connected to Projector on 120" screen, with iPhone + iFi iDSD DAC connected to line-in of Receiver. Controlling Roon on iPhone via Harmony BT Remote. 
20210328_190542 by Hilton, on Flickr
20210328_184247 by Hilton, on Flickr
iPad BT Harmony Remote by Hilton, on Flickr
iPad BT Harmony Remote by Hilton, on Flickr
iPad BT Harmony Remote by Hilton, on Flickr
iPad BT Harmony Remote by Hilton, on Flickr
W.E.T. - Retransmission by Hilton, on Flickr
Also iPad Mini Remote with Backlit BT Keyboard.
20210328_201306 by Hilton, on Flickr
iFi micro iDSD Signature TH610 by Hilton, on Flickr
20210328_221816 by Hilton, on Flickr
Emby Theatre 4K HDR.
