Create clean install of MacOS, yet still have access to old install
My goal is to create a clean install of MacOS, yet still have access
  to the old install while I work on the new install. I am assuming that
  I will miss some items initially and will need to refer back to the
  older install.


This is the very essence of a (full) Time Machine backup and the proper application of Migration Assistant.

Another option is to use an external USB drive and make an image of your existing drive to that external drive.  Once you are certain that you can boot from it and operate properly, you can then do a clean installation on your internal drive.

To make a clone of your drive, you can use a commercial application like Carbon Copy Cloner or use dd in Terminal to make the copy:

dd if=/dev/diskX of=/dev/diskY bs=1M


Where X is your source (internal) disk identifier and Y is your target (USB) disk identifier. 

Whether you use a commercial piece of software or the terminal command you need to make sure your target disk is the same size or larger than the source.