2019 iMac as display/dual display monitor

A zero cost solution is to simply us Screen Sharing via ethernet.

Share the screen of another Mac - Apple Support

Depending on your usage it may work well enough and

cost no more than just time to try.

No it's not possible.

FWIW, there is a device available for sale which supposedly would allow it. Whether it works well or not I can't say. Never tried it.

https://lunadisplay.com/

FWIW, Screen Sharing is too slow for my needs so I didn't mention it. However, it may be fine for yours.

What? Not even with your Cat 8 infrastructure? 🤓

lol I do happen to have one Cat 8 cable running to one iMac. Not sure that counts as an infrastructure. 😎