Thanks for your comments so far.
I’ll be buying an extra 2 x 16 GB of RAM, which will give me 40 GB total.
Regarding the SSD, please excuse my ignorance if this is a dumb question. Is the speed benefit of the SSD purely due to having the OS and applications installed there? If that’s the case, would having the OS and applications installed on my external SSD and booting from that give me the same benefit?
As a secondary (maybe dumb) question, as far as I understand the regularly used files are copied to RAM, so if I have 40 GB of RAM this would be more than enough to hold the OS files, Photoshop application files and image data - so would a large internal SSD (rather than the smaller amount on a Fusion drive) actually make much difference?
Thanks again for your help.
The speed benefit of the SSD is not only purely do to having the OS and applications in there, but the main reason is that the OS uses the main boot disk as its swap disk. Ideally to have good performance, you should allocate about 50Gb of free space. The best performance I found is 100Gb of free space even if you have lots of RAM. And the reason is that you are not only loading the image files into memory; it only includes copies of the images you use with the filter (smart filters) in Photoshop in non-destructive editing and the app and OS may determine that even 40Gb of RAM is still not enough. I used to work in the digital imaging industry for close to 30 years before I left for a different career, but before I left I work with the best top photographers likened like Joe Mcnally (I was with Nikon) and they all use maxed out computers with tons of ram. Which was why I said 64Gb ram is the best so you don't and never will encounter any slowdown in your workflow. Also, you need to remember that if you plan to use this new machine for a few years, apps and needs and features are coming out using AI (Artificial Intelligence for image editing) and that would require even MORE ram than you are planning right now. Which is also why those newer Mac Pros can go up to 128Gb ram and people who need them do max them out. But in your case, I think you can start with 32Gb and that would be fine.
In regards to disk space, I would highly recommend that you divide your storage in 4 separate categories. First category is your boot drive; this will contain your main OS, the apps you plan to run and a large enough free swap space for 50Gb to 100Gb storage. I would not recommend a fusion drive, because of its smaller SSD size (I think up to 128Gb of SSD in 1Tb of storage?!?). Anyhow, the fusion is only fast when everything is accessed through its built-in SSD, but slows down when some things are on the HD platter, thus negating your performance advantage of your faster computer. A fusion drive is meant to be a hybrid option for those non-power users who would benefit from faster boot times, but still enjoy the large storage single space. These people don't usually run multiple storage options anyhow.
The second category of your storage option is a "scratch disk". This disk is dedicated for apps like Photoshop or other imaging apps that can use another drive other than the boot drive to prevent certain tasks from being bogged down. This scratch disk ideally should be a very fast SSD drive with fast write speeds and with RAM cache (slower SSD does not have RAM cache and they are a lot cheaper). Or if you have lots of ram (like 64Gb RAM of memory) and you only need 40Gb, you can allocate the remaining 24Gb as a super fast RAM disk for scratch and filters purpose. RAM disk is much faster than any external storage options because you are accessing RAM internally, plus you don't need to do any trimming with RAM disk, because once you turn off your computer, the content in the RAM disk will be gone. Speeds up your disk management workflow as well.
You want to separate the scratch disk from your boot drive as you don't want both Photoshop and the OS to be swapping files on the same drive thus bogging down your performance. The third storage category is the media storage drive. This drive exclusively stores your media (your multiple sources of original RAWs) and can be a small SSD drive, but big enough to accommodate all your RAWs for your workflow at the moment. You want to focus on buying the SSD drive with a faster read specs and large RAM cache on either a TB3 external case or a USB 3 external HD dock to help with read speed. I personally use the USB external dock so I can swap the media drives (for photo or video editing). Then lastly, the fourth category is the media storage drive. This can be the slowest drive in your workflow; like a RAID HD array or a single large HD. This is where you store your finished work/workflow. In my case, I have 2 RAID 4 HD storage array both backing up each other so I have triple redundancy protection (Raid 5 + 2 RAID boxes so if the RAID box fails, the other RAID box should still be ok).
Never focus all your work on just 1 or 2 drives, because if 1 drive crashes (happened to me a number of times over a span of my career in digital imaging), you're screwed if you have all your work in just one basket especially if you're dealing with super large PSD files.
