"PLANNING TO CLEAN-INSTALL:
MacOS Mojave (I will only go this far, so I can still run iPhoto"
WAIT.
STOP RIGHT THERE.
There's something you need to know.
You CAN STILL RUN iPHOTO with the latest versions of the Mac OS, and this includes Monterey.
All you need is the free "Retroactive" app that patches iPhoto, iTunes, and Aperture so that they'll run on the newer OS's (that Apple claims they won't run on).
Having said that...
Here's how I'd recommend you upgrade to Mojave:
1. Get an external drive
2. Use either CarbonCopyCloner or SuperDuper to clone the contents of your internal drive to the external drive. BOTH of these are FREE to download and use for 30 days, "doing things my way" will cost you nothing.
3. You will need to get a copy of the Mojave installer. If you're having trouble getting it from Apple, I recommend you use this free app:
IGNORE the part about "your Mac is already supported".
All you need this utility for, is to download Mojave.
Go to the Tools menu -- there it is.
4. You will need to make a bootable USB flash drive. It needs to be a 16gb or larger drive (I don't think 8gb will do).
5. To make the flashdrive, I recommend using either "Install disk creator" or "DiskMaker X". Again, both are free. If you have problems with one of them, TRY THE OTHER.
6. Boot from the USB flashdrive.
7. Open disk utility and BE SURE to go to the view menu and choose "show all devices".
8. In the list on the left, click on the SSD and erase it to APFS, GUID partition format.
9. Quit disk utility and open the OS installer. Start clicking through.
10. The iMac will restart one or more times, and the display will go dark for a minute or two with no other indication of activity, so be patient.
11. When done, you'll see the initial setup screen (choose your language)
12. Start clicking through. When asked if you wish to migrate from another drive, YES, you want to do this.
13. Connect the backup and let setup assistant "digest" everything. It takes a little time.
14. Setup assistant will present you with a list of things that can be migrated. I suggest you choose everything and just "let it go".
15. This will take a little time. When done, you should see your login screen. So log in and look around. All the apps "should be there".