A few years ago I was going to Crete, straight across the Alps from Swizerland, with the Alpine route being direct from Wangen-Lachen LSPV to Corfu LGKR.
I found tons of stuff on "mountain flying" which, to most people, seem to mean flying inside the Alpine canyons, often below cloud, and the emphasis is to avoid the mountain waves and other turbulence, while not getting into a canyon too narrow to turn around in.
However I was going straight over the top of the Alps. As is normal in VFR, a route had to be planned OCAS and in this case it was FL130 because above that was Swiss Class C airspace (and as it turned out the Swiss would not let me into it) so I would be flying only about 1500ft above the mountain peaks. It was obviously important for there to not be much turbulence due to wind.
I found very little material on how high one needs to be to do this. The best I found was that one needs to be 1000ft above the peaks, for every 10kt of wind flowing across them, to be sure any downdraught is no bigger than a few hundred fpm. It has proved to be a good rule.
Ovee Wales etc it works OK because there isn't much low level airspace there so one can fly high.
On a recent flight in Spain I was crossing the Pyrenees, on a track of about 040, towards Biarritz LFBZ, from Granada LEGR. There was about 20kt of wind flowing across the terrain. I could not get above 17500ft no matter what I did, whereas normally the TB20 will go to 20,000 without any great trouble. Quite obviously there was a descending airflow of about -200fpm... not much but bad enough if trying to stay above the cloud and above icing. In this case the terrain was about 8,000ft below!
The above was an IFR flight so not limited by CAS but I would never again fly across mountains VFR, when limited by airspace above, through which a transit can potentially be denied. It's a way to get snookered, and ATC won't give a damn until you declare a mayday.