Not sure if this is still pertinent but here is my process.
1 - I have multiple backups, Time Machine and iDrive (cloud backup service), so if anything went wrong I wouldn’t lose anything.
2 - I copied everything I wanted from my iMac internal HDD to an external HDD - Documents, Photos library, Music folder, etc. I made sure everything was there. Re: Apple Photos —- I keep my full library on my Mac (full size versions), not the optimized library.
3 - I installed High Sierra on the external SSD. During installation I logged into my Apple account to restore my settings etc. I did not use the Migration Assistant. Note: I did not allow iCloud to sync my photos at this time because I did not want 120 GB of photos downloaded to the SSD.
4 - I started High Sierra from the SDD and made sure it worked properly.
5 - I reformatted the iMac internal HDD. I wanted to start from an empty HDD.
6 - Moved everything from the external HDD (see step 2) to the newly formatted iMac internal HDD.
7 - I opened the Apple Photos library from the iMac internal HDD (open from the library itself, not the Photos app). Went to settings and checked “icloud photo library” and “download originals to this Mac”.
8 - After you are sure Apple Photos works properly and it is using the library on the iMac internal HDD, you can delete the library that Apple made in your Pictures folder on the SSD.
9 - Opened iTunes and indicated which folder on the iMac internal HDD to use.
10 - If you want downloads to go to your iMac HDD, you can indicate a folder in your web browser settings.
11 - At some point, should you need to clear out space on the SSD, you can move more stuff to the iMac HDD.
The main things: 1 - Backup everything before you start. 2 - Be cautious at every step, but if you are not, refer to 1.
Good luck with whichever method you use.