alright, thx for all Your help and patience - ;-) - I now have my desktop still on GhostBSD-25.02 (xorg), but my Toshiba Portege is now on 26.1 with xlibre-server and fully working (well, no VirtualBox so far) plus my Thinkpad T520 is on 26.1, but with upgraded version, keeping xorg as server.
Guess, VirtualBox with xlibre-additions (as suggested by Eric) would work, as I tried a simple VM installation of 'Syllable', but I ll keep testing this on the T520 and xorg. More RAM space and disc space there for virtual machines.
My end result is: it was a mix from Bash as (preferred) shell plus the wrong VirtualBox-packages, that caused my LightDM freezings with no mouse & keyboard, starting with the upgrade to p8. Now staying on zsh and delegating the VM things to my Thinkpad, all updates worked well and I ll try to keep it that way on the Toshiba for some time.
T520 now my testbed for everything - if it goes wrong, I ll re-install from scratch. :-)
For other testings and models, pls have a look on my other post 'installation via USB stick...'