Didn't the OP say his wife cleans around where the iMac is and has misplaced his external hard drives?Ive done the same on my 2011, I am currently using a Lacie TB that I put a 512 SanDisk SSD in. Its running 11.6.5 perfectly. Very usable machine in 2022.
Dude should just take it to a third party computer shop, have them install an internal SSD.