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The previous Zeb air spring would be one step forward (more usable travel) and three backward (worse quality of travel. Hollow mid stroke and harsh initial travel)

The cartridge design of the 38 air spring is intended exactly to allow increasing both the negative and positive chambers so yes, that makes it significantly less progressive. I don't think there's much difference in the lower leg ramp, if any at all. While the 38 uses the volume between the inner cylinder and the stanchion as part of the...

And a comparison with the Fox 38:

Can someone explain to me why the very low pressure would affect the progression? I don't fully understand that. Technically the progression is always the same regardless of pressure. As you said, at lower pressure you have more access to full travel but the mid stroke is also less supportive. So the difference between the mid and end stroke is...

The Luft Fusion mentioned above should be almost perfect for you. Not just for the mid stroke support but your weight the decreased compression in the lower leg should be noticeable. I haven't measured it on the zeb but on the lyrik debonair C1 at 160mm the difference is 8-10 kg at bottom out (just for the lower leg). On...

So did you find also the new boxxer too much progressive?

The 38 uses the space between the inner tube and the stanchion to increase the lower leg volume. This also allowed to move lower the seal head, slightly increasing the neg.volume. The Luft Fusion linked above used all the space between the inner tube and the stanchion to double the neg.volume. The trade off was that by using a smaller...