It should show something more like;
(base) [dean@MeshifyC-Manjaro ~]$ pacdiff -o
in a well maintained system.
This of course is your choice; certainly, reinstalling is probably the fastest route to a working system. Then, you need to set everything up again, just the way you like it. I suppose that what I’m trying to say here is that it’s likely going to take time, either way.
The .pacnew files should best have been addressed as they came to be. Skimming through that list, some of them might have even had system security implications.
Going forward, whether you choose to reinstall, or not, you should certainly pay more attention to system maintenance.
Make a habit of checking the latest Update Announcements for any condition that might affect you; the issues and solutions are generally listed for anyone who will take the time to read them.
When using a rolling release distribution, the expectation is that you (the system admin) will take a more hands-on approach.
However, if that’s not you, there is a new edition of Manjaro in the pipeline that might be more appealing. For the time being it’s called Manjaro Immutable; if you search the forum, you will find a few references to it; like this one, for example:
Cheers.