Exploring options and alternatives to buy a new monitor vs trading brand new mac mini for imac.
My spouse and I both use iMacs for Lightroom CC and PhotoshopCC for image editing. We are both ardent amateurs, not working pros.
They work fine with the Retina displays - get plenty of Ram, at least 16 Gb these days I think, and plan on some kind of external storage media with several back ups.
My spouse uses external hard drives via USB 3.0, and I use an external RAID 5 array. Both work. Ultimately it depends on your needs and your budget.
I previously had used a large LG monitor that I still like a lot.
The new Mac mini has a lot of potential for CPU power and RAM if configured that way. I am thinking it's my next desktop. The one weakness of the Mac mini is the built-in integrated graphics (Intel UHD Graphics 630), is weaker than on the MacBook Pros. This can be addressed by adding a Thunderbolt eGPU if you have the space. But that is not cheap, and the iMac has a better GPU built in.
I think the Mac mini integrated GPU is enough if you spend all your time in Photoshop and Lightroom (or similar) with one monitor. That's why I'm still thinking of getting a new Mac mini to go along with the great monitor I already have. It's really the video editor/3D/multiple 4K monitor types who need a better GPU. But if you do add a Thunderbolt eGPU to a current Mac mini, you can bring it within striking distance of the more powerful Macs on applications that actually make use of a GPU.
Whether you switch to an iMac might depend on what monitor you would be willing to get instead. Since I have an accurate wide gamut NEC with some features that an iMac monitor doesn't have, I don't want an iMac, I just want to connect my nice monitor to a Mini. Similarly, if you are looking at maybe buying a monitor that exceeds iMac features, you can just keep the mini.
If an iMac monitor would be a definite upgrade from what you have now or are thinking of buying, swapping the mini for the iMac could be a good move and you would get a GPU upgrade too, as long as you stay away from the cheapest iMac, don't skimp on RAM, and if the price of all that is still OK.