Use iMac as a Monitor for M1Max MBP?

[kphelan]

I just received my 16" MBP M1Max, and want to use it for all my LR etc processing.  I have been using a late 2017 27" iMac for processing.  I would like to try using the iMac as a monitor for the MBP.  I tried the obvious, connecting a usb-c cable between the two, and then tried "add a new monitor" to the M1 MBP.  It does not see the iMac.  The reverse is true however.  The iMac will add the M1 screen as an additional screen. 

Does anyone know if this is possible?  I'm trying not to buy a new monitor, but if I must I will.  Looking closely at a BenQ SW271C, currently on sale at $1499.  Any advice much appreciated.
 
Thanks,
Kent

[kentphelan]

UPDATE:  This is not possible per Apple support.  Certain older iMacs can be used as monitors (in conjunction with also using High Sierra).  A bridge too far for me.

https://support.apple.com/en-us/HT204592

Will probably buy the BenQ SW271C.