My understanding is that most Electronic Control modules on BMWs have a start-up self test. The system monitors that all inputs are within range. If it detects something is out of spec it will turn off the functionality of the system. So, a brief moment of it turning on and then off probably means the self test failed but the ECH is alive (as you know from communicating with it).

Given you don't have bags installed, I take it there is no pressure in the accumulator and pump is disconnected? If the ECH cannot start the pump to get the pressure up, it would turn off the system. Also, if it detects the car is not at the ride height it demands but cannot adjust, the system assumes a fault and turns off. So, I think you are on the way but without completing the install it will be hard to guess where any issues start from.

__________________
2003 4.4i black ext./black int./black headliner (kid's runabout)
2014 535d X-drive, M package, silver/black interior

Sold but not forgotten:
2009 E70 35d, black ext./black int./black headliner (sold 2021)
2006 4.8iS Le Mans blue/cream int./black headliner, SOLD in 2012 sadly...

Other hardware:
2015 Cayenne S (wife's new DD and definitely wroooommmmm)
2016 Tundra 1794 edition crew cab, luxo hauler
2005 Tundra crew cab, weekend hauler. Sold after 150k very happy miles.