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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.
Didn't the OP say his wife cleans around where the iMac is and has misplaced his external hard drives?

Dude should just take it to a third party computer shop, have them install an internal SSD.
 
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