Jeff, as to the modification date of the library:
the system version below your post is stating macOS 10.15. Which version is it exactly? Did you already update to macOS 10.15.4? In the early versions of Catalina the "date modified" and the total size of the Photos Library did not update reliably. For me, the problem with the library size seems to be fixed in macOS 10.15.4, but the date modified is still wrong. I am working daily with the library and import new photos, but it does not change the modification date.

I did look at PowerPhotos and it seems that creating a separate library of those old pics I want to archive on a separate disk would be a great way to go and worth the purchase price and learning a new app.
It will work perfectly, if you do not enable "Optimize Storage" for your iCloud Photos Library.
I am using Power Photos a lot. It is just great for several reasons. I like it most for giving me a second browser for my Photos Library, where I can browse the photos as a table with the metadata arranged in columns. I can search for duplicate photos, and it is easy to create partial libraries as a backup of the new photos.