iMac processor upgrade

You can use it as a display but there are limitations Use your iMac as a display with target display mode - Apple Support

I sold my late 2009 iMac on Ebay a little over a year ago and profited about 1/2 the price of a refurbished iMac from Apple. You might be surprised how much you can get for a good used iMac.

The reality is that iMacs are not meant to be "upgradeable" in the way you would like. Some Macs can accept different processors, but the reality is your iMac cannot go farther than MacOS 10.15 Catalina. I've upgraded a lot of iMacs over the years and other than SSD's and RAM, there's not much else to do on them.

That being said, if your iMac is working fine, upgrade it to Catalina and continue using it. While BigSur is visually different than Catalina, for most folks it's not really that different when using it day-to-day.

iMacs do have decent resale value in the private market, but your your iMac is already 9 years old and consider it a testament to Apple's quality that you have a computer working for you that long. A new iMac, along with a 5K display will have you wishing you did it sooner.

Just know that the rumor mill has Apple doing a full refresh of iMacs this year using the new Apple-Silicon CPU's with an entirely new design. So know that going into this. If I were you, I'd upgrade your iMac to Catalina enjoy it until the new iMacs are released.

iMacs aren't meant to be upgradable beyond storage and ram. Your best bet would be to sell it and buy the iMac you need.