I think that you might be better served upgrading your iMac rather than add a second monitor to a system already compromised by age.
I purchased an iMac 27" i9 late 2019 unit and it outperforms my $6,000 2017 iMac Pro for half the cost. Supposedly, Apple will be refreshing the iMac with the Apple-made M1X processor to replace Intel processors. This should be a rocking system. I'm considering replacing my iMacPro with this system after it comes out and I see some reviews.
CaptureOne Pro has been my image processor of choice for the past 4 years. The latest version 21, works great on both of my iMacs.
Both units are hard wire connected to my 10GBe LAN. The iMacPro has an internal 10GBe ethernet connection. For the iMac, I purchased the SonnetTech external Thunderbolt 2 10GBe ethernet adapter. Read and write speeds to my DiskStation DS-1819+ on my 10GBe LAN average between 550 to 700 MB/S for real-time back ups of my image library. The DiskStation has a 10GBe ethernet adapter card installed.
For my iMac 27" i9, I have the Apple-approved LG 27" monitor attached and it works great. Both the iMac and LG monitors calibrate very well with my 6 year old ColorMunki.
Regards,
Bud James
Please check out my fine art and travel photography at
www.budjames.photography or on Instagram at
www.instagram.com/budjamesphoto