Cool that it works for you. Be happy
To tell the end of the story: after the next oil change the problem, severe dragging, was back
I talked to someone who had a similar problem and he said I should try a special engine flush oil. I asked him why and he said, after I informed him that I had replaced all parts of the clutch except the hydraulic line, that he thinks the Motul V300 I put the clutch discs in prior to installation has done something (chemical reaction of the Ester contained in the oil) weird. It did sound plausible as on my bike the clutch lift was the same as on my buds bike, but on his the friction plates opened up easily and on my bike they stuck together and needed a bit of force to free up.
After I flushed the engine and re-filled fresh ATF it was better but not perfect. A strange thing was that my clutch pull didn't feel right, it was kinda sticky. Finally I traded the bike in for a new one and of course another GasGas.
I admit, that in 40 years of spannering on almost everything that has a motor, including jet engines it is the very first time that I don't have the slightest idea what was going on nor do I have the slightest idea what the root cause was...