If you're needing a heavy stable lifter... I'd agree, the M600Pro is an excellent option. I have 2, and they are so stable... I relate a comparison to a smooth Bomber to an agile Fighter. Probably the best stable controllable platform, and easiest to see at a distance.
I'd agree, a smaller package can meet similar quality of the large platforms in many ways, even exceed. A Mavic 3 Cine is a nice craft compared to an
Inspire 2 X5S in many ways. Although the
I2 offers multiple lenses, CinDNG, Removable SSD's and a X7 FullFrame S35 that's still out fount as leader. Plus as you've indicated, using SDK and 3rd Party Apps takes an earlier craft like the
I2, MP2, and Matrice series.
If you're only seeking to mount up a X5, that's a light load for the M600Pro... and just a note if going X5, the X5R is superior in multiple ways. For X5 MFT Payloads, the
Inspire 2 X5S would be a good choice... both on Lightbridge 2, and the X5S is a bit better than X5R but extra on licenses that are free with X5R: ProRes & CinDNG.
The Ronin MX is a nice gimble mount for larger payloads, and the Gremsy Gimbals: T or S series are a notch up, faster setup, faster payload balance and still in supported production. I have a Ronin MX planning to move to Gremsy. Mount up a Sony A7series, RED, or other and you're gaining on the payload for Cinematography, Modeling, Maps, etc.
Not attempting to discourage your M600Pro decision... it's a wonderful platform with multiple tangents of operation.
The price target might be on the low side, especially if more than 1 set of Batteries. But as always, depends on the Seller... if they're more wanting to sell than retain, price may be lower. I casually watch eBay prices and they appear to be appreciating not depreciating recently.
Best of Luck on your treasure hunt and hope you enjoy it... Great Craft.