Firstly - Welcome to the forums and secondly, that's a great 3 to start and gives you quite a range of tonal options and variety.
Your collection is similar to my first 3 PRS guitars - the 594 (like your SC), Custom 24 (like your P24) and 509 (my alternative to a strat - a 'Super Strat' in essence - like your SS). My next guitar was my Hollowbody ii - partly for the Piezo (which you already have) and partly because its a Hollowbody. My 5th was my Special which is a Semi Hollow and a more modern, with much more tonal options, double humbucker with trem guitar - more modern than my 594 offers anyway. I notice none of your PRS double Humbucker guitars offer a trem bridge - not sure if you want one either but maybe something you may consider for your next??.
When you have those 3 that you do, assuming you look to expand without too much crossover - guitars that add something your others don't, it does get a bit tougher. For example, the Paul's guitar - a double 'humbucker' with individual splitting options - could be covered in my collection by my 594 (I know its different sounding) or my Special (again different sounding) but I wonder if its different enough to justify the cost in my head. I don't have P90's so that could be an option (if I liked P90's). At the moment though, I am GASsing for the 594 HB - I know my HBii has the same PU's but no splitting options (I could live without) and Piezo instead - obviously the 594 HB offers a different shorter scale length and the Pattern Vintage neck but there is a LOT of crossover but I love the PRS Hollowbody guitars and a marriage between the 594 and a HBii is a marriage made in heaven as far as I am concerned. If Winter NAMM and/or the PRS experience, which often sees a special guitar made for it, doesn't see a Guitar that offers a lot to my Tonal palette and catches my eye like the 594 HB, then the 594 HB is likely to be next in my Collection.
What do you think you will be adding next? Do you have other guitars that you think covers areas that affects your decision - like having a 335 so not really looking at adding a PRS Hollowbody (either the 594 or HBii)? Do you have guitars that you may consider trading in for a PRS alternative - even an Acoustic? Its clear that you have a good range of guitars to cover a wide array of tonal options so where do you see your PRS journey going? Based on what you have, I would look at either a Hollowbody (perhaps the 594 unless you want another Piezo although I believe its different on a solid guitar) or perhaps the PRS Special Semi-Hollow - partly because its a semi-hollow with a LOT of tonal options and a Trem and partly because its a Limited Edition and may be harder to find as time goes on. I know you are not me though so I look forward to hearing which direction, which model you may be considering next.
If its not in your plans right now, I am sure these forums will show that inevitably, you will add more at some point LOL
Anyway Welcome...