10.13.6. Safari keeps crashing, Can't use Google Marketplace with it. Can use Chrome but been with Safari for years. I'm not sure if the Mac is slow ot it's the internet.
Have since found out that I cannot use a 2018 Mac Mini, so I bought a 2014 one instead, only to find that Target Display Mode works from the 24" iMac upwards. Mine is 21.5.....so will have to have patience till I can afford to upgrade my iMac