Malte Hi Malte, good info to work with - my experience with seeing kite foilers going to winging is that they adapt super quick and are anyway used to much smaller boards/more nervous foils etc. So I think you could easily go to the 5'0", as you mention super light winds are not your thing. 5'0" you can probably kneel start too, so no issue there. And, finally, most importantly, you probably have more chance to get your hands on a 5'0" - the 5'4" has massive back orders right now, country depending.
The 5'4" is designed as the "advanced" board for someone coming from bigger sizes, for early planing. But for riders going out in stronger winds/chop/downwinders/wave, the 4'8/5'0" are ideal. I use the 5'4" myself a lot here in southern Germany, on the lakes, mainly with a 6.0 Echo/1750 or 1500 Wing, am 78kg´s and just one yr older than you! 😀 but not that skilled in general foiling, although 38yrs windsurfing experience. If I was to go to the coast or ride more often in stronger winds, then I would go for the 5'0" too!