Hey Klaus, I’m really sorry for missing your message, I’ll have a look for it. (I don’t see all posts, the idea here is to have players talking to each other but I check in every so often)
I really suggest watch all of the Largo Stage videos with a fresh mid and then watch the PU-LIP-KU-KMT-TU 5 articulations video.
YOU DO NOT HAVE PHYSICAL LIMITS from what you have said.
The tissue (find airstream), visualiser (place rim), mouthpiece, leadpipe, sympathetic oscillation, leadpipe/trumpet transition, will all get you into the low register really efficiently.
I think re-looking at these video will really help.