I suggest that the "stay" is actually designed to stop the door opening past 90 degrees to thus prevent the door/farme/hinges becoming damaged. I am not aware of a "stay" designed for a full size door which is designed to hold the door at any fixed position between closed and the 90 degree position. A skiding block as described can only work in one position…….fully opened. Having sufficient friction to resist wind pressure, or allow easy opening/closing on a sliding stay is just about impossible. Again I suggest that the renedy should be provided by the suppliers. After all AJW when you asked for "stays" did you agree exactly what the fitting was supposed to achieve. the sliding device you have described is a stay designed to hold the door at 90 degrees.
Incidentally the multiple position "restrictor stay" suggested earlier is designed for windows, and nor full size doors. It will give a retricted opening of probably not much more that 100-150 mm for ventilation without jeopardising security.
Edited By MadMike on 12/08/2020 22:47:21