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I don't think you'd need to partition an SSD, for the same reason we don't defrag SSDs. They handle all that organising the blocks of data so much better than a mechanical HDD.
I have a Windows system with 64Gb of RAM, and so far, Photoshop has never once created a Temporary file on the Scratch drive, which is a 512Gb Samsung 960PRO NVMe drive. The biggest document I tested this with was a 30,000 x 30,000 pixel document we use for testing performance (by timing a 5000 pixel brush with 1% spacing to move from corner to corner (11 seconds on my i9-7900X system)
I don't worry about reserving drives for Scratch space when using an SSD anyway, because of how they work.
But if you do use an external drive, how will it be connected. I am out of date with Mac stuff, and never knew much anyway, but I remember reading that Thunderbolt was uber fast. My gut feeling, has for several years now, told me that anything that requires a lot of disk activity slows Photoshop down. Even 4000 pixel square documents can get into lag with 150 layers — especially when you start nesting Smart Objects.
We'd appreciate it if you come back and give us a heads up on how your new Mac does with Photoshop, and maybe try the 30k pixel 5k pixel brush test, but with 64Gb RAM, I really don't think you have too much to worry about.
Thanks, it seems that going with an internal drive to begin with is the best choice, and then I can always add an external one if this is not sufficient.
Will report back with experiences when the time comes!