iMac runs like a wounded dog, no desire to have a Fusion Drive anymore

Thanks, Dan, I had already watched the guide for how to do it.

I wasn't sure if leaving it in would cause problems when installing MacOSX - I didn't want it creating a Fusion Drive again.

As the SSD I have ordered is the same capacity as the HDD (1TB) I assume it will be clear enough when creating a new system which is the target SSD and which is the HDD? I will want to format both before I do anything else - and I hope it won't boot from the system on the HDD before I get a chance to do that.

Also, will there be any complications with Time Machine trying to back up the HDD as well as the SSD? I assume it will automatically only use the SSD as the source.

@Fred Farnackle - To make things easier I would create a bootable USB drive either USB-A or USB-C OS installer drive (32GB is all you need) before you start!

Grab the OS installer file from here How to get old versions of macOS or from the Apps Store if you have the newer. Make sure the drive is formatted with GUID and HFS+ (now called Mac OS Extended (Journaled) in the newer Disk Utility). Then follow this How to create a bootable installer for macOS. If you have an external drive case you could place the new SSD within it and then format it as well and even give it a name so you know the drive by name. Here's a deeper writeup on Disk Utility How to Format a Mac Hard Drive With Disk Utility

Your system should boot up with it present using the Startup Manager Option (⌥) command after checking it before you start. Now proceed with the upgrade.

Reference: Mac startup key combinations

Thanks, I followed the instructions for creating a USB boot drive with Monterey on it. It is all as expected when the key combo is pressed at start and it fires up.

Since I won't be creating a Fusion drive, do I still need to format the SSD as APFS?

@Fred Farnackle - I just use Mac OS Extended (Journaled) on all drives. The OS installer will convert the boot drive as needed. In the end the boot volume will be APFS with the newer macOS releases.

Now you are wondering why not just go with APFS on the booting drive when formatting it? The OS installer will undo it as it needs to make a few hidden volumes besides the primary OS volume. In the end its' a bit more work for the installer program.