Perhaps you may consider an external drive to run your iMac?
That may require certain unusual steps or processes, to work.
OWC offers examples, external drives (Envoy etc) enclosed units.
https://duckduckgo.com/?t=ftsa&q=owc+how+to+use+external+drive+to+run+iMac&ia=web
Using an External as Your Startup Drive: Part 1 - macsales / owc
https://eshop.macsales.com/blog/55907-using-an-external-drive-as-your-startup-drive-part-1/
OWC offers internal HDD/SSD replacements for iMac specific upgrades.
Upgrades Guaranteed to work with your iMac by OWC
https://eshop.macsales.com/upgrades/imac
Late 2012 iMac 27-inch, etc
https://eshop.macsales.com/upgrades/imac13-1
https://eshop.macsales.com/upgrades/imac13-2
Or make appointment for repair AASP or discuss with Genius
at retail location? For possible ideas on how to replace Fusion
drive, now that's either defective or broken. Had you tried to
restore the 'two drives' so Fusion would work as One?
Apple Repair and Repair Status Check - Official Apple Support
//support.apple.com/repair
Genius Bar Reservation and Apple Support Options - Apple
//www.apple.com/retail/geniusbar/ [hardware/software option]
The options vary by intensity and efforts to resolve do too.
Not all AASP would replace an internal drive with OWC. IF
they did, probably might order it themselves; and markup.
Usually a display comes off to access internals, incl battery.
