I found many posts about this online and in this forum, but not for my configuration, which seems to matter. I want to use a mid-2011 Mac as a 2nd display for my mid-2017 Mac.
• Main Mac: iMac, Mid-2017, Core i5, 27-inch, 3.8, Mohave 10.14.4, Thunderbolt 3 (USB-C)
• 2nd Mac: Mac, Mid-2011, Core...