Move MacOS from SSD Back To iMac Disk

I have 2.4TB and rising of my wife's professional photos on our iMac - a lot of RAW files chews the space up. The iMac is 2019 and has a 3TB disk and had pretty much ground to a halt.

I took a suggestion from here and moved the iMac startup disk and user files to a LaCie 1TB SSD and it works fine. Someone jumped on the thread and said I had done it the wrong way around and I should have put the photos onto a drive. When my wife presented me with her MacBook with another 500GB of photos, I realised the commenter was right.

I have now bought a 12TB SanDisk pro external drive and all the photos live there. I then set out to reverse the process and put the startup disk back onto the iMac. The issue is I can't see the target drive on the iMac if I use Migration Assistant.

When I go into Disk Utility, I see the disk (actually disks), but I can't find it any other way. See the screengrab.

Any advice, please?

In a perfect world I would simply get a bigger desktop, but the disk space on iMac has topped out and the next models up are $$$$$$ and beyond what we need.