Thank you all for your answers.
I just dismounted the tyre, and weighted all parts with my electronic balance (by weighting myself with and without parts).
The 2 PLY TA tyre weights about 2.1kg
The tube : Wheel TA 2.25-31 : 600gm (original 36" tube I guess)
Hub+rim+48 pokes : 2.1kg
Before dismounting the Tyre, when I inflated it and then listen to hear where was the leak it seamed to come from the valve, and when I was touching/bending it I could clearly hear the air leak getting stronger.
But once the tube retired from the tyre, pumped a bit and plunged under water, I couldn’t see any bubble anywhere.
I just can see some of the tube matter above the boundaries of the valve disk plate detaching itself a bit, but not to the point that I see a hole in the tube.
I presume that the air only goes out when the PSI are above what I can put in the tube without any tyre.
Anyway, what do you guys think about the weight of the tyre, tube and more interestingly the wheel’s weight (hub+spokes+rim) ?
I guess I can save a few hundreds of grams by buying a Nightrider tyre and a Foss 36er tube (being able to inflate the tyre to 65PSI without doubling the max recommended PSI would be a big + ), but I can’t do anything about the 48 spokes and the rim itself.
So if it really makes the wheel more easy to accelerate and control, it definitely worths it and I could use this old Qu-Ax for a while before thinking about selling it and buying a nimbus or kh 36er, but if a new tyre and light tube doesn’t change anything, it’s not the same deal.
By the way, the space between the rim sides is about 27-28mm,the part of each side in contact with the tyre bead is about 5mm deep, the rim has a U form on the tube side, and a _/ form on the spokes side (see the photo) I just hope it can keep the nightrider tyre beads tight at 65 PSI.

Since it is not a 4PLY TA but a 2PSI TA (the mystery is still here), I’m not sure I want to inflate this TA to the double of the max recommended PSI.
Another question about the nigthrider tyre:
is there an old and heavy Nightrider tyre and a recent one lighter, (I guess so) and if yes, what are the release date of the new one?
I ask this cause I’m not sure the french unicycle web shops have the last one, but if I know the release date I could ask more precisely before ordering it.
Edit:
Very interesting.
I can’t wait to read what are your feelings with the High PSI