shower pan leveling

Get the pan level and fasten it securely. Prevent bending or breaking by filling the space under it with something reasonably solid. Shims would leave gaps into which the heels of the average adult human would push the pan's material.

Your dimensions work out to 192 cubic inches, or a little over a tenth of a cubic foot. Volume of five or six caulking gun tubes. RTV should be solid enough if surrounded so it can't bulge out. Harder substances preferred. But you'll need tubes to squirt it farthest from the opening, so you don't trap air behind it.

Or carefully mix 192 cubic inches of filler, pour it in, then push the pan into it. Push a little at a time so as to get it level. Let the stuff harden, then remove any leakage. (Most of which will squeeze into the raised border of the pan.)

I had a shower pan that rocked (wiggled) back and forth as you stepped on it. What I did--got the base level and then sprayed underneath it with foam (from a can, similar to what is used around electrical boxes etc). It expanded completely filling the space under the base and then oozed out of the base. When it hardened I just trimmed all the stuff outside of the base. Worked great, base solid as a rock. MLD

A variation on Option 1, unless this is what you meant by "level concrete slab"...

Pour a bed of mortar and press the pan into it, making it level. Use enough mortar to fill the voids below the pan and make it feel very solid. If you've ever stepped into a fiberglass shower or tub with voids underneath and felt the give, you'll know what you are trying to avoid.

shower pan installed (32in x 32 in) already installed but on a slope. It is about 3\8 in. out of level. My odc prevents me from not correcting this even though it does drain. Three options.

yet. Just pan with surrounding studs. Pan has not been affixed to studs.

I'd go after the fiberglass ribs. Less likely to settle than redoing the slab.

WBT, you have lysdexia in addition to your ODC.

It is normal to set fiberglass tubs and bases in wet mortar.

enter this: set fiberglass tub in mortar bed in google. YOu will see plenty of advice.

On second thought, leave the pan as is, and just relocate the drain.

So you don't have to build up the sides.

about 3/8th off level? I'd find something else to worry about.