Update: I tried importing the XML file again and let it run for 12 hours and all in all like 12 albums downloaded. No playlists or anything.
So then I went to my Mojave 13" MPB "Go" computer (older secondary computer that one I don't keep anything on, just use when I'm out for basic tasks) and imported all my music and other iTunes media. Once that was all set up, I did the upgrade to Catalina and all my songs were all there, my playlists were there, play counts, etc. Also, the new TV+ app and podcasts apps were all setup correctly and pulling files from the iTunes media folders where they resided (still within the Music folder).
So then I tried manually moving over the library files for Music and TV+ to my new 16" w/ Catalina. That kind of worked. I did have to do an import from the XML file which this time worked fast. But because of my two failed attempts at import I had a lot of duplicates I had to clean out. Not too bad. What was annoying was that my TV+ wasn't accessing *all* my files. The "Home Videos" tab was missing which is all my vids that I've had imported into iTunes over the years so that I could watch them on my iPod.
My Podcasts app wasn't accessing my saved podcasts in the iTunes Media folder as well. Neither of these were a terribly big deal but if it could all be done right, I wanted it to.
So I tried plan B: Which was to try and clean install Mojave on my new 16" MBP, setup iTunes and everything on that, upgrade to Catalina, and then with these files setup right, import the rest of my files back from backup. So I cleared the hard drive of the 16" MBP, made the Mojave installer, and went to install, but:
I looked around on the web and it seems on the 16" you can't do a Mojave install. So the was out.
The last option then was Catalina install again and this time bring all the iTunes, TV+, Podcast stuff via the Migration assistant. That process worked.
And as I'm typing this, I'm bringing all my files back on from back-up.
I lucked out having a computer with Mojave on it. I was just too lazy to do the upgrade on that for however long Catalina's been out. Looks like my laziness paid off.