use iMac as external display for MacBook pro

This Apple article shows you iMac too new to support Target Display Mode:

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

There are third-party options but some are expensive and all have some serious lagginess. One purveyor of such products goes as far to include this on their support page:

Basicaly you get a place to park static palettes and windows.

For the cost of some of the less-capable options you can buy a very nice free-standing external monitor with full option, high resolution, and no lagginess. It will also use less power than a running iMac.

Maybe, using third-party solutions such as Luna Display. But with external monitors so cheap, it probably won't be worth the hassle and compromises.

Regards.