Hello Nicolas,
thank you very much for reading my post and your kind answer. I had a look at xpra and it does look very interesting. Though it does not fit my current user case. Did you try the Xspice setup with Debian 9? When you set up /etc/preferences like I described (and have the sid sources in /etc/sources.lis) and do:
apt-get -t sid install xserver-xspice
it will only pull that package from sid and not mess up a stable Debian install at all. I would love to hear how if you get the same results that I do.
Regards,
Malte