HI Gisbert and welcome.
You have some nice ideas here. I think the reason you need to run twice is that the cue is sent b efore the sync is set up, and therefore missed the first time round. If you move the cue line to the end and put a small delay before calling it all will b e well on the first run.
#moved to the end
sleep 0.01 #small delay to make sure that sync is waiting before the cues is issued.
#Choosing the variation to begin with
cue [:ua, :va, :wa, :xa, :ya, :za].choose