Well the resolution of your linked monitor is 3440x1440 (not 5k) which is much lower than the 27” iMacs 5120×2880. So again you’re going for a lower resolution on a much bigger screen.
I absolutely wouldn’t go less than full 4k. So I’d stay away from anything that says QHD. If i was going ultrawide I would need to have a vertical resolution of at least 2160 if I was moving from a Retina display. (Something like the LG UltraWide 34WK95U) I would still be stepping down pixel density so would be losing sharpness but I would probably cope as the display will still be fairly sharp.
The retina displays are so good that to match / beat their sharpness on a bigger form factor you really need to go 8k or go for the apple xdr display at around £5k. People look at the iMac’s and think they’re expensive but they are actually incredibly good value for money particularly given the quality of the display. So…whilst you are right in saying you could trade in your iMac and get a Mac mini and screen for the same money, you will be taking a major step down in image quality. I think to get closer, you’re going to have to pay out a bit. To match / beat it, you’re going to have to pay out ‘a lot’
Don’t get me wrong, there are advantages to going bigger or ultrawide and I don’t regret my decision to go for the 32un880 which is full 4k (but still not retina) It’s just important you understand the trade offs. The other thing to factor in is that you probably need to be investing in speakers and a webcam. Whilst monitors will likely have speakers built in, they’ll be a bit **** in comparison to the iMacs. My lg monitor does not have a built in webcam.