Extended Display

Nope, last iMac you could use was a 2014 Non-Retina iMac...

• 27-inch iMac models introduced in 2009 and 2010: Use a Mini DisplayPort cable to connect your iMac to a Mini DisplayPort or Thunderbolt or Thunderbolt 2 port on the other Mac.
• All iMac models introduced in 2011, 2012, 2013, and mid 2014: Use a Thunderbolt cable to connect your iMac to a Thunderbolt or Thunderbolt 2 port on the other Mac. If the other Mac uses Thunderbolt 3, you can use the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter with your cable.

Target display mode requires macOS High Sierra or earlier on the iMac used as the external display. It does not work with macOS Mojave or later.

I understand, you under purchased because you did not thoroughly research what you ordered. You can speed up the computer by adding an external USB 3 SSD. The RAM could be upgraded, but by and independent AASP not an Apple Store. However I would skip that and just get the USB 3 SSD. Then clone your internal HD (use SuperDuper! or Carbon Copy Cloner) the internal HD to the SSD and then use the SSD as your boot drive.

Doing the above SSD upgrade will dramatically improve performance of the computer and have no affect on the warranty or AppleCare.

Nope, only curtain 2009 thru 2014 model iMac's support Target Display Mode.

see > Use your iMac as a display with target display mode - Apple Support

Nope, it's impossible. If you need an external display it is time to buy one.

Your iMac is a computer and was never designed to be an external display for another computer so I don't understand your point of view thinking the iMac was a waste of an investment.

But good luck.