Update: The culprit for CPU spikes, 504 Gateway Errors and general havoc was due to a plugin: Timely All-in-One Event Calendar.
I have disabled it on my staging sites, but still need to find a good alternative for the live site.
I feel like a jerk in putting @weaver & @scrambler through all this. Yes, there was an initial problem with updating theme to 5.0x with my header and menu bar width, but that was resolved before this thread started.
Thank you so much for your support and patience.
What a crazy way to start off the new year.
And in time, will look into Cloudflare.