Did you disable LightDM before enabling SDDM? If not, that was likely the reason for your crash.
Looking through your notes (above) it seems you brought most of that grief upon yourself – blindly following ChatGPT “suggestions” and random posts, with no clear idea of what you were doing – but, rest assured, you’re not the only one. 
Booting with a Manjaro Installer USB and entering a chroot environment is an established method of performing maintenance outside of the running system. This should never be considered a last resort, but a necessary tool; users are encouraged to always have a very recent (if not the latest) USB on hand.
I’m not convinced that it is a driver issue, but then, I made a conscious decision over a decade ago to use AMD graphics exclusively, which means I’m rather out of touch with Nvidia of late. That their drivers are still giving grief to a major portion of Linux users is no surprise, however.
I seems like you have taken the wrong message away from that topic. If you had read much past the first few posts, you might have noticed several wanings about the procedures suggested;
More importantly, you missed the warnings from a Manjaro Team member:
And regarding official drivers downloaded directly from Nvidia;
If I recall, when installing another driver there were prompt(s) to replace/uninstall the previous – again, I haven’t used Nvidia in some time, so treat this with caution, and seek clarification.
The following Wiki page typically has all the information needed to install the best available drivers; rather obvious by it’s absence, you’ll notice that “downgrading” isn’t mentioned at all, for good reason:
There is also this – noticed in a recent Update announcement topic:
I presume this choice is given during install when a supported Nvidia GPU is used; and/or when updated versions of these series are made available through a normal update session.
I’m not aware of whether a new procedure exists for changing the driver series, post install. This might be something others could expand on.