If you aren't comfortable prying the iMac open (there is definitely a risk of breaking things inside during a drive upgrade), you could do this:
Get an external USB3.1 gen2 SSD, such as:
- Samsung t7 shield
- Crucial X-9
- or buy an nvme blade SSD, and put it into the USB3.1 gen2 enclosure of your choice.
Then, migrate your internal setup (you could use CarbonCopyCloner or SuperDuper) to the external SSD...
Then, set it to be the new boot drive using the startup disk settings pane, and then...
... boot and run that way.
USB3.1 gen2 on an external SSD will give you read speeds around 850-900MBps or even a little better at times. Quite snappy.
Change the display?
If both the display AND the drive are giving you problems, it's probably time to be looking for a replacement.