If all devices are disconnected and G5 won't run from DVD-Drive, what influence should a change of SSD have?

Diag-CD has 2 modes: OF and OSX-mode.
If system passes OF mode, then there are no hardware issue detected. This normaly means your hardware is OK!! But this is also known bei booting into a working OF-console - which works too (in your case).

2nd test is OSX-mode: This will be interessting, if this works or not. In this case drivers for all devices are overtaken by OSX (OF-drivers are not longer active) and some functionalities are enabled, witch OF does'nt. If this is working, then your issue is very strange.....

One thing witch OF does'nt test during boot, are different resolutions of your graphics-card - but OSX and MorphOS does! Maybe there is an issue......