So i have success to enable wayland with
But after i have all the Freecad windows and text oversize scramble…so i have the memory i have ever add this problem and i found the post wich give me the trick 
QT_AUTO_SCREEN_SCALE_FACTOR=0 QT_SCALE_FACTOR=1. freecad
So then i have Freecad gui ok on Wayland…
But always the problem of very bad save on file…
So i compare my directory of a new user account where Freecad run and mine…
And i found a in home/xxx/.config directory a qt5ct directory ? curious because i have reinstall qt5ct and remove it with the pacman -Rsc to delete config file but always a directory…
so because you have show me thats the freecad crash have says i have a qt5ct theme use and the problem is about comming from this theme manager I delete the qt5ct directory i restart Freecad and test again…and this is working…now my Freecad work like a charm for save. and This is ok on wayland or X11 mode …Many many thanks again to have show me the Qt5ct abnormalyti use .
And now all my Kde app start 3 time fast …wonderfull…this qt5ct have been add complet distorsion on my users profile…very curious.
Have a nice days. long life to Manjaro.