Bruno, our plane side weak link never touches the ground. After a couple of tows dragging the glider side weak link on the ground, the tow side is now stronger. I can see the difference in the glider side weak link.

Yes we use a clear vinyl tube over the weak link rope to keep the ring from chafing the rope.
The Janus C specifies a stronger link, 16xx # I think, the 5/16" rope made into a weak link covers that.
Sometimes we run. 5/16" link on the tow end and for all the other gliders a 1/4" link on the glider end.
From a weak memory I think the 1/4" weak link rope is 1200# strength.

We tired the braided poly, but it did not stand up to our rough conditions. So we stay with 3 ply rope.
We are very good at eye splices. Any solo student can make a respectable weak link.

We also use whiffle balls to keep the whip and sag of the rope at a lower rate.

BillT