Hi, I know this is a long shot but is there any third party RTK system out there compatible with the M600 Pro?

Alternatively, has anyone used the D RTK-2 Broadcast Mode with the M600?

If you speak about GNSS receiver of the drone:
The DJI CAN bus only seems to accept DJI accessories. Therefore you can only use the DJI RTK module (D-RTK). I removed the A3Pro from mine and replaced with a pixhawk for these reasons.
D-RTK - Product Information - DJI

This may be interesting :
D-RTK install on M600 Pro with PICTURES and VIDEO | DJI FORUM

If you're talking about the Base Station:
D-RTK onboard doesnt accept anything else than D-RTK base station. I do not know a way to use it with third party NTRIP provider.

the DRTK2 is a GNSS receiver which has extremely inefficient software, and anyway not compatible with D-RTK for M600.

LV_Forestry Posted at 7-5 12:49
If you speak about GNSS receiver of the drone:
The DJI CAN bus only seems to accept DJI accessories. Therefore you can only use the DJI RTK module (D-RTK). I removed the A3Pro from mine and replaced with a pixhawk for these reasons.
D-RTK - Product Information - DJI

Thanks @LV_Forestry
I've been thinking the same of putting a cube in the M600, and an inspire 1 gathering dust...