Hi everyone,
I finally made up my mind and installed YSS shock absorbers with the same wheelbase (390 mm) as the originals (model TE302-390T-05-X): what changed?
Practically nothing...

The shock absorbers work well and, in certain situations, even slightly better, but it's a minor thing.
The stiffness in the rear is probably caused by other factors (but I won't go into it).
Positive aspects:
- preload adjustment with the threaded ring: allows for more precise sag adjustment;
- spring distance: in the unfortunate event of a deep pothole that causes the shock absorbers to "pack," with the YSS shock absorbers, the bump stopper is hit first rather than the springs as happens with the original shock absorbers (which can cause back pain).
I forgot: the tightening torque of the original ones (iron) is 40 Nm. Since the lower fork of the YSS shock absorbers is made of aluminum, I preferred to be cautious and reduce the torque by about 30% (I tightened them to 30 Nm) and I had no problems.