>> I have a friend with a 2011 27" iMac that uses thunderbolt 1. It has an absurdly slow HDD.

Almost every single one of those that we bought, had a hard drive failure. They used WD Green drives. I replaced them with WD Blacks, and if SSD's had been cheaper in 2012/13 we would have used them, but these were graphics designers.

You will need to control the fans with software like SMC Fan Control, which is annoying, or better yet, this harness: https://eshop.macsales.com/item/OWC/DIDIMACHDD11/

You can pop the glass off an iMac with a credit card (or your fingernails, they don't glue them on that year, instead they used magnets) and with a torx bit you can tilt the screen out of the way and pull the bad drive out.