After half a year of fiddling I finally found a insane workaround.
I hope posting links is ok on this forum?! If not, please just delete this post.
I find it ridiculous to be forced to setup such a workaround just to get acceptable data to skydemon, but at least it works. What still is weird is, that skydemon shows different altitudes with my Holux-Receiver and with my internal one, but I will figure that out as well even if I would prefer skydemon to read the data form my internal GPS-receiver natively.