I agree with Ascension. However, don't try smoothing it with gaussian blur. Don't.
At least not everywhere.
I'm not sure what the options are with ps5. Is that Paint Shop or a really old version of Photoshop?
Anyway, here's what I'd do. Take your elevations that you used to create that hillshade and paste it into a new document. We're working with elevations, here! Greyscale, BABY!!!
Now create a new empty layer. Set the blend mode on that layer to multiply.
Choose a pretty large and really soft brush. I like grungy brushes with a lot of jitter effects, but I'm not sure what your app is up to, so a big soft brush is golden.
With the opacity on the brush set low, and the foreground color set to a fairly dark grey, paint down the lowland areas where you don't want hills and mountains. Ten strokes at low opacity is infinitely better than one stroke at high opacity. We want to hide our fingerprints.
NOTE: If, for some reason, you can't set the opacity on your brush, get another app! No, wait! If you can't lower the opacity just use a lighter shade of grey. You may have to go to darker shades as you move away from the mountains, but it should work. Kind of.
You could have done all of this with a brush set on multiply over the original bumpmap, but this is less destructive. Doing this on its own layer also allows another little trick to hide the artifacts of your editing yet further. On that multiply layer run a blur filter, preferably gaussian blur on a fairly wide setting. If you use too much just undo and try again with a smaller radius. If you don't see any effect, try again with a larger radius.
Now, try your lighting effect, or hillshade or bumpmap or whatever the effect or filter is called on ps5, using your newly fixed elevations. See how the mountains and hills pop out and the flatlands are... um, flat.