And both the thread you mentioned and the link posted here have explicit instructions for what to do there depending on your intentions.
splash enables ‘boot splash’ (what plymouth does).
quiet hides output.
Removing both would make booting more verbose and should also temporarily disable plymouth.
There are also extra options for explicitly disabling plymouth.
3 sets runlevel 3.
Placing a 3 among the options disables gfx and boots into console.
(and so on)
So …
You could first try replacing quiet splash with
plymouth.enable=0 disablehooks=plymouth
Which may work.
If not … you might try just the 3 to try to get an interactive console.
If none of that works … you will probably need to use a live-usb and manjaro-chroot.