Can I connect my Mac Mini to my iMac as a first monitor?

No. Not with your recent Mac Mini.

The requirements for Target Display Mode are set out in Use your iMac as a display with target display mode in which both Macs used in the process must be old models.

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

presciously few imacs models support the feature of acting as a monitor

and newer macs seem to have lost the ability to use any imac as monitor

this product seem to offer some of it

Luna Display | Turn your Mac or iPad into a second display (astropad.com)

though not sure if it would work as the only monitor

No, however you can connect most any external display or even an HDMI TV to the Mac mini.

have a look at > Connect a display to Mac mini - Apple Support