Either one or both of the drive elements of the Fusion drive has probably had a hardware failure.
Have a look at this: Use an external SSD as your startup disk … - Apple Community
Maybe that is what you have been doing already.
If you want to keep using your 2017 iMac and boot from an internal drive, I think your best path forward is to have an Apple Authorized Service Provider install a new 1 TB SSD and deliver it back to you booting properly from that drive with the latest MacOS the computer will take. This is what I did for my wife's old 2015 iMac several years ago, the SSD cost about $100 and labor was about $50. It's running fine, it runs Monterey, yours can run Ventura, which is actually not bad at all.
It may be that the cost of doing this has increased since I did it to my wife's iMac (she has a new Silicon iMac now of course, I get to use the 2015 model). In which case you should look at the costs and pros/cons of these options:
(1) Buy a new iMac -- these start at $1250
(2) Have AASP install a new SSD in the 2017 iMac -- lower cost, but you have then a 7-year old Mac that is will never get past Ventura, which is now two generations old in MacOS and more software and web sites will stop working with it as time goes on. Also, the 2017 computer may have other aging/degrading hardware components.
(3) Use the 2017 iMac booting from an external SSD. Lowest cost option, has all the disadvantages of (2). I think this is a viable option given that an iMac is a desktop machine anyway (I would never suggest this for a laptop).
I don't think you should waste time trying to figure out what failed in the Fusion drive and how to repair it. The mechanical drive in the Fusion drive is a vulnerable part and I would never want to replace it with another Fusion drive.
