Advice wanted on (possible) transition from iMac to Mac Mini

I’m considering replacing my 2010 High Sierra iMac with a 2020 M1 Mac Mini (or a new iMac). There are some potential complications and preparatory tasks:

  1. The iMac has a 256 GB internal SSD (replaced stock ca. 2015) and a 2 TB internal HD (with about .5 TB free). I don’t want to pay for a massive internal SSD on the Mini, probably would get a 512 GB.
  2. But…I have several important folders symlinked from the SSD to the HD (Dropbox, Music, Photos, maybe 1-2 others) - this seems much more breakable if I have the HD as an external drive (thinking I would use the two drives in the current iMac as externals).
  3. My iTunes library on the iMac has always been “canonical” i.e. it has all my original ripped CDs while my other devices use Apple Music. What would happen with this in the upgrade / transfer from High Sierra to Big Sur? I’m not sure yet whether I’d use Migration Assistant (if it works from HS to BS) or manually bring stuff over.
  4. I have a ton of photos in Aperture in the iMac which I’d need to move to Apple Photos or elsewhere (fortunately all referenced files) - doing this would probably make my iCloud Photos library too big to keep a copy on my MBP or iPad Pro.
  5. I can’t see the LG Ultrafine 4k in person - how does it compare to the new retina iMac screens?

Any advice, suggestions, warnings etc. are most welcome.

Regarding (1) and (2), I have a Samsung T5 SSD. Not crazy-expensive, stays plugged in all the time, never had an issue. Very fast file access over USB-C.

I have had USB 2.0 drives take a while to connect at boot, and that can cause issues for things that absolutely must function at boot time. But I don’t think I’ve had that issue with the T5.

Here’s the internal HD I have, formatted as Mac OS Extended (Journaled):

and here’s the internal SSD (APFS):

I was hoping to just find cases for them to convert them to external drives.

Everything is up and running on the new Mac Mini, so I wanted to share a quick update. With the system, Plex, and OneDrive running, there are only two things that I wish I could change…1) I probably should have went with 16GB RAM; 2) I wish I could purchase/afford a 4TB SSD external drive…:). It is connected to a Dell 38” Ultrawide monitor via USB-C and a 27” Asus 1080p monitor via HDMI and both look great.

The machine runs great for the scope I wanted it to cover (a Plex and file server). However, after setting things up, I really want to use it for my basic office tasks as well. While it seems that I can do both most of the times, the RAM does get close to capacity when I have Outlook, Word, PowerPoint, and a couple Excel files open, but doesn’t really seem to slow it down even with Plex Server and OneDrive running in the background.

I set the system up as a new machine and transferred things over by connecting through the network with the old High Sierra machine and the Mac Mini both on Ethernet. I look forward to using it more.