OP here. Here is what Puget Systems (a very respected PC company) says about choice of boot drive for use with Photoshop editing:
"What type of storage drive should I use for Photoshop?
NVMe is the absolute best storage medium for your boot drive, your swap drive, your application drive - on every computer, every day of the week. You work against that NVMe drive when you do your initial dump of photos from a trip - is that the performance you're trying to re-create for your entire photo library? An external SATA SSD is 1/4 to 1/3 the speed of your internal drive.
I'm glad you've taken my advise to not use an external drive as your boot drive.
Nothing I have told you is in disagreement with Puget Systems's recommendation, and I'm going to remind you that you're working with a 'luxury' computer. I'm not recommending NVMe for the 6tb of storage, and I suggested the cheapest method of improving things is by adjusting your workflow. Since that doesn't work for you, there are options but the cost can be all over the board. A single 8tb drive, splitting to 2x 4tb solo drives or even 4x 2tb drives with SoftRAID. Or a Lacie Big or Drobo, not that i would personally use either. A small NAS with 10gbE would also be an option, but I wouldn't recommend that for your setup.
You're at that unfortunate position of being in the 5-10tb of storage required, so not fitting into a normal SSD, but also not needing a large multi-drive array with higher capacity. There are 8tb SSD drives, but I don't recommend them due to the cost per GB, and the all your eggs in a single drive.
Flip thru other threads on storage here on LuLa and you'll see my opinions for others.