I noticed that the reason VM page hang is because my plex docker setting includes the gpu even though GPU is not in used that time. The moment I shut plex down, my VM page works => this is the root cause
As for the force stop VM via terminal, I still don't have a solution that doesn't include system reboot / shutdown.
Maybe as of 6.9.2, it's not possible. But I do hope this can be a future feature. Afterall slackware OS is still Linux-based, terminal should be able to do everything, considering that the default terminal user is already root user.