Martin, thank you so much for clearing that up. It's the internal workings that I can only guess at!
Now I just have to decide whether to buy an external SSD or temporarily move things to the Fusion drive and re-evaluate -- I guess I can copy the libraries and just find out, then delete if it doesn't help (much).
I'm hesitant to buy another external drive just because I don't like daisy-chaining -- it's the main reason I freaked when I had to go from a MacPro to an iMac (the so-called "trash-can" edition of the MacPro requires external drives so had limited appeal to me when my 2010 model died).
I could, however, "time-share" one port with the Time Machine drive, which I usually turn off during recording anyway (except when I forget -- so this would be a better way to remember!).
I appreciate you giving such a complete yet concise answer. I think it will help others besides myself.