Is it possible that i could just use the screen from my Retina 5k and not the Mac part. Connect the new Mac mini or Studio to it?
It is possible, and this is what I do for home use, but there's a big "but".
You need to open it up, gut the Mac part out and reconnect the screen and backlight to a driver board (you can get these direct from China via Ali Express) and rebuild it all into the case. This isn't simple like building a PC, you need to work out how to mount it all and there's no real documentation to speak of. This is a kinda "if you have to ask how it's probably a bit tricky" situation.
On the mini vs studio, given you can't immediately say why you need the studio I'm 100% certain the studio is the wrong Mac for you. The M4 was such a step up over the M3, let alone the M2 series in the current studio, that unless you know exactly why you need the extra ram or memory bandwidth in the top end studio then the M4 or M4 pro will be faster.
Here's a synthetic benchmark suite of the _base model_ m4 (not pro) mini vs a top end M2 Ultra studio:
browser.geekbench.com
The M2 Ultra only wins on multi-core stuff, but not by much. Compare to a M4 pro mini and it's all over for the studio:
https://browser.geekbench.com/v6/cpu/compare/10053310?baseline=10103270
A base m4 mini vs a M2 Ultra studio is a win for the base m4 mini
https://browser.geekbench.com/v6/cpu/compare/10053307?baseline=10116800
Or to put it another way. For most use cases, you need to spend £4200 on a Mac Studio to beat the £600 mini. If I spec a mini to the sam ram and disk as the studio (64gb/1Tb, needs the M4 pro, and 10Gb ethernet to be fair there too) it's now £2300 and faster than the studio for nearly everything, you can buy a studio display to go with it AND STILL SAVE £400 vs the studio.
Viewing 100% I get a much better idea of the actual sharpness and detail of the image, and while it's possible to scale the screen,
I just have Lightroom set to zoom to 200% when I click in on an image. Provided you're not doing fractional scaling more pixels on the monitor will always be better for evaluating sharpness etc. A 5k screen is still only 218 DPI, you'd print at 300 or 600. On a windows machine don't scale the whole screen, you can set it to be rendering 1:1 and then scale all the fonts/controls up. Then you get sharp text etc and no funny scaling on images etc.
I do see the advantage of lower res screens for photo work, where budget is limited you can probably get a better colour screen for the same money with lower res. And budget is nearly always a limit for home use.