Thanks for the assistance, both of you. CMOS battery replacement is a good idea, but not what fixed my issue here. I ended up turning AHCI on instead of RAID, and disabling the SATA drive slots I wasn't using. Not sure exactly which fixed it, but now my 9020 boots up seamlessly.
As above, I have successfully installed and configured an M.2 SSD as the boot drive on my Dell Optiplex 9020 MT, and after some trial and error, everything is working great. That said, every time I boot the PC I get a warning saying no boot devices were found, and to press F1 to continue, F2 for setup, F3 for diagnostics. If I just press F1, the OS boots normally.  In boot order options in the BIOS, my PC is set up to UEFI but only one boot order option is listed: Windows Boot Manager. What do I need to do to get rid of the error on startup?
Optiplex 9020, upgraded and booting from PCIE SSD, but getting a "no boot 
devices found" error on startup