Update from me on my rig
@Sniffer , I went for the OWC 1M2 external drive caddy as recommended above. To my absolute amazement, a 4TB NVME turned up in my Christmas stocking (which was nice!). Installation took about 5 minutes and the only complaint I have is the permanently on white LED which is blooming bright at night.
Mac Mini wise, I settled for the 512GB storage / 24GB memory model. I don't do video editing either so LR and Lr Classic are about the heftiest apps I run routinely and the Mini remains stone cold and the 1M2 probably a little warmer than my hand. I hope I'm not teaching anyone to suck eggs here when I mention that you can conserve internal drive space for apps by offloading even core Apple stuff such as your Music and picture library to external disk.
I've also loaded up the ARM version of Windows 11 under VMware Fusion for the rare occasion that I need Windows stuff for work but I haven't stressed that yet, it was just a test to see if works and it does.
The final nail in the coffin of my old 27" iMac was it routinely spinning up it's fan to full tilt whilst using video conferencing software such as Zoom or Teams which I do frequently every day for work. It could be argued that I've paid an Apple 'hardware tax' but it has been worth it as all my apps now run happily on the current hardware / MacOS combination.
I can't defend my purchase of the Apple Studio Display financially but I'm glad I did, the speakers are frankly good enough for me to consider retiring the pair of Sonos One's that sit on the desk and the integrated camera (although criticised elsewhere) is a significant improvement on the old iMac.
My one possible (but not absolute) criticism is that despite what we read, I don't think the standard M4 Mini is up to gaming. As an experiment, I. downloaded the xPlane 12 flight simulator demo and boy, oh, boy, does that get the Mini hot. So much so that I abandoned the test.
All in all, I couldn't be happier. The iMac is packed away in the loft in case of emergency and a possible conversion to a 5K screen if I ever work up the courage to separate the screen from the chassis and install the various gubbins from eBay to do the conversion.
All the best,