I have the same model You can increase the resolution in preferences, display but there is no 5K setting. Also it would be so small it would be difficult to read. From published articles https://everymac.com/systems/apple/imac/imac-aluminum-tapered-edge-faq/how-to-run-imac-retina-5k-at-full-resolution.html
The Retina 5K iMac models from the "Late 2014," "Mid-2015," "Late 2015," "Mid-2017," "2019" and "2020" lines all have a 5120x2880 native resolution, but by default run "pixel doubled" at a setting that looks like 2560x1440, it just has four times the detail of a "traditional" display.