Sorry just not possible ;-{ Here's some background:
* [https://www.tekrevue.com/target-display-mode-imac-2017/|Target Display Mode Remains Absent from 2017 iMac - Update]
* [https://www.tekrevue.com/tip/ins-outs-imacs-target-display-mode/|The Ins and Outs of Your iMac’s Target Display Mode]
The only thing I can think of is remote desktop but it won’t offer the performance you are looking for [https://support.apple.com/kb/PH25556?locale=en_US|macOS Sierra: Allow Remote Desktop to access your computer] and [https://docs.microsoft.com/en-us/windows-server/remote/remote-desktop-services/clients/remote-desktop-mac|Get started with Remote Desktop on Mac].
Best answer here is a second display as much as you don’t want it.