Then, I put this:
https://www.amazon.com/OWC-Accelsior-PCIe-SATA-Adapter/dp/B0...
And this:
SAMSUNG 850 PRO 2.5" 1TB SATA III 3-D Vertical Internal Solid State Drive (SSD) MZ-7KE1T0BW
In it as my system drive. The volume looks external (its got that orange and white icon), but everything has worked perfectly for about 18 months. It boots just fine.
For me, the performance increase with this change was incredible. Its like a new computer. I my OS and all my installed applications on it. I keep all my data on traditional drives in the other four bays. Its like a brand new computer. It was completely worth it. Right now, I use it for very intensive workloads of various kind, and never even think about replacing it.
Finally, you can find a lot of info online about whether you should enable TRIM or not. FWIW, I did do it, and its been working fine for 18 months:
https://www.cnet.com/how-to/installing-ssd-on-mac-trim-mista...
I cannot remember at this point how I copied the data over. I do remember that whatever method I found on google worked quite quickly, and that there were no hassles with it. After doing some kind of copy operation, I booted to the new drive and never went back.
Hope this helps.