Thanks for this, Adam. A very good read,
I use a similar set up: one catalog (yep, essential) with just over 45,000 images. As you say, this makes it easily searchable. And then the YY-MM-DD-subject file structure.
You mention renumbering to make sure there aren't two files with the same number. One thing I have found helpful: I'm fortunate enough to have 3 Nikon bodies all of which produce a DSC prefix to the file names. I've renamed them in the camera's default setting to DSZ6, DSD5 and DSD500. This helps a bit.
On backups, like you I have set up a RAID system. In addition to the main copy on my iMac (with a 3TB fusion drive), I have 3 RAID copies, two in one drive enclose (a G-Drive system) and third which I swap out once a month and which is then stored in my basement. I have found the software package GoodSync excellent for keeping al the copies the same. I also have 2x 4TB drives in a Synology NAS which updates very hour or two. So that's, errrr, 6 copies in total! Obsessive or what?
Thanks for pointing out the "Import from Another Catalog" option. Just goes to show you, that you can use LR for many years without spotting something!