I have a similar problem, but discovered if I’m really patient, it ends up booting. On my laptop I wait for the system to go to sleep, then I tap the power button and the login screen appears (still on 6.4.4). I described this more here.
It would be interesting to see if the behavior is the same for you, if you wait a long time (leave it overnight?) does waking the laptop up give you the log in screen.
Like you, none of the the recommended ways to get debug info worked for me. It appears to freeze up before getting the chance. (Do you just get a single underscore on the screen?)