dear community,
thank you so much for all your ideas and hints!
by now, I have got it working in the desired fashion by just using qjackctl as has been explained by Rui Nuno Capela some days ago and again yesterday by Christopher Arndt, i. e. simply by using a script within qjackctl and its patchbay/connections manager. 
the reason why it didn't work for me was just that I am not so familiar with the syntax, I didn't know about the '&'.
but now it works very well.

thanks again, have a good day you all!
christoph

Am 31. August 2019 22:14:40 MESZ schrieb Christopher Arndt <chris@chrisarndt.de>:
Am 27.08.19 um 17:58 schrieb Rui Nuno Capela:
also, and this is pretty much mandatory if you're doing it from one of
the qjackctl script launchers, always remember to append an ampersand
('&') to the end of the command string, eg.:

 Execute script on startup:    [ killall qmidinet ]
 Execute script after startup: [ qmidinet &       ]

And then use the qjackctl patchbay or something like jack-matchmaker,
aj-snapshot or jack-plumbing to establish the JACK connections dynamically.


Chris