My understanding is the double-declutching you describe (i.e. blipping in neutral with clutch up) was only needed for old pre-synchromesh gearboxes and not required for modern ones like in Z3s.
My process:
1. brake
2. clutch in (still braking)
3. change gear and blip throttle as stick passes over neutral (clutch still in, still braking)
4. clutch up (still braking)
5. either come off brake and accelerate away; or repeat steps 2 to 4 if needing to grab another gear
I can't (haven't mastered) #3. If I were block changing, I would definitely double de-clutch, but I can't do it under control of both brakes and throttle.
Re #3 - unless the clutch is dragging, there is no benefit doing the blip in neutral. That blip could be done at any time the clutch is in.
I've worked out the difference in what we are doing
and why...
You are trying to increase the engine speed to match the new speed of the gearbox to balance the car.
I am trying to increase the speed of the gearbox input shaft to reduce wear to the baulk rings.
The blip in neutral (clutch up) turns the gearbox input shaft at a speed that is closer to the new speed of the main (& lay) shaft(s). It is doing the same as you are, but with the additional advantage of getting the input shaft in the gearbox turning at engine speed, so only the main (& lay) shaft(s) needs to match the output shaft speed.
It's a habit that I got into in the 80s.. You are correct in that it is not
needed. The gearbox will handle "normal" gear changes, but I've replaced baulk rings on gearboxes in the past. I don't want to do it again (especially on a ZF350). If I can make my gearbox last as long as the engine, I'll be a happy bunny (or penguin

). Having a lightened clutch doesn't help, especially as it is a "normal" flywheel, not a DMF.
I find it very difficult to drive in an unsympathetic way. I find it very hard to slide the car. If I wheel-spin, it is nearly always because I have lost control. I drive like Miss Daisy before everything is warmed up.