Some interesting ideas, thanks for taking the time. Is there a penalty for having multiple operations accessing a single SSD at the same time, vs having multiple (smaller) SSDs?
That's the specific thing I'm trying to avoid - I don't want to do any manual management of files and space, which is what I was hoping Fusion Drives would do - keep active files, ie anything new, opened or saved, in the ssd portion, while keeping my volumes simple.
I suppose I can start with a 240gb ssd for OS and apps, leave my user directory on the spinner, and see how that goes.
That's easy, I will still keep the same strategy, a single SSD. 500GB size should be good enough for you.
Always import photos to the SSD, and use CCC automatically copy all photos to the photos HDD on regular basis (e.g. everyday).
So, HDD 1 is a clone, if a photo is removed from the SSD, the same photo will be automatically remove on the next clone.
And HDD 2 is NOT a clone, but keep all photos until full. Then automatically remove the oldest photos when space required for the new photos.
Yes, multi smaller SSDs operation in parallel should provide better performance. However, I can't see why it will become a huge bottleneck in your case. The SSD latency is so low, and normal users rarely require > 3 operations at the same time. Especially the overall file size you are dealing with is not that large. Yes, coping 30GB photos still take some time. However, by considering everything now on the same SSD, no need to copy it from one SSD / FD to another. It actually save time.
Since you sleep your machine at night, that will Very good to use CCC. Let CCC wake up the machine at mid night. Do all the clones and copies. Then sleep the machine again. So, every morning. Your Mac is ready to work. All you need to do is just import photos to the SSD and do your work on it. No need to take care the HDDs.
Also, on every morning, you are safe to delete all the old stuff on the SSD, they are all backups onto the backup boot drive + photos drive (daily backup, photos only) + time machine (hourly backup).