You need to get rid of the particle board completely, sorry but you can't install anything over which may leak water !

He doesn't have particle board, but OSB. This is an acceptable substrate for a styrofoam shower pan.

I agree with Jaz, use the 3/8" plywood, cut the appropriate size furring strips based on on your slope at the perimeter of the shower pan, then screw the plywood down at perimeter and at shower drain,to create your pre-slope, even though your floor is not level, you should be close enough to pass inspection after your shower pan material is installed

Similar but different problem...

I have a shower in a basement that was being remodeled when I bought the house. Shower is studded out and covered with cement board. Structure is very sturdy and with the cement board, it’s very solid. The only problem is, I noticed when I put the level on it that some of the areas are not completely level. I don’t want to tear it all out so I thought I could correct. The most it’s off is about ½ in a few places. One option I thought of would be to mud all the walls, but that might be over kill. So, could I do one or two walls to correct the problem? Or can I fix it with thin set? And how?

My next issue is waterproofing, I can fix some of the joints but would feel a lot better just applying Kerdi to the surface to give me peace of mind that everything is sealed. Can I do this if you suggest I use mud on the walls to fix the alignment? I still have to complete the mud deck since it is just a pipe coming out of a concrete floor. Since it is custom shower size I can’t us a pre made pan.

Any help would be appreciated.

Furring out a tapered wall with mud is best left to the pros, even for us it's a difficult task. Far easier to take off the cement board and sister all the studs so that he new ones are plumb. If the cement board is screwed on this should only take a couple of hours. you could also cut tapered furring strips, but at $2 or 3 for a 2x4 it hardly seems worth it.

I agree with Jaz, use the 3/8" plywood, this will work, you can also use the plywood as your pre slope if you install furring strips at the shower perimeter according to the slope required.