Well I have to agree about Grub Customizer. It sure didn't work for me. I tried what you said about going to Z16 lite and reinstalling grub but I got a message that Grub couldn't be found.
So I did another search on this forum for reinstalling grub and found this thread:
I followed your directions to get to the Grub file and then edited it for the default boot OS. I saved the edit and updated Grub and now my PC boots into Z16 lite just like I wanted it to. I had looked at that thread before but didn't really understand as much as I do now. Thanks for your help. I will mark your post as the solution since it led me to the correct solution.