Both the Hollowbody (especially with Piezo) and Silver Sky are quite different from a 335 and Strat although the Silver Sky is closer than the HB is to a 335. The Solver Sky is a 63/64 strat and individually treated when it comes to the electronics to ensure they are the same. Not every SS has the same capacitors (I believe). I can understand though why you thunk you have the Strat covered but I do think the HB is different enough and with the added Piezo offers more than other HB guitars. Both worth looking at though.
The Custom 24 is a quintessential PRS. Its the guitar that launched PRS to the world and the most PRS sounding guitar. Its a modern sounding and feeling double humbucker instrument and comes with the thinner neck compared to the others. Its not Wizard thin though and not that much thinner than a regular - not enough for me to be overly different but different enough to noticeable when compared. Obviously, being a 24 fret neck and joining the neck at around the 22nd fret, you can reach the 22nd fret easier than a standard 22fret guitar but that's a minor difference in my opinion but for some, that's still useful. The big difference though is really the position of the bridge and how that changes the neck tone - making it a bit brighter than a 22 fret with the same PU's. The Custom 22 is more like a Modern LP type with a trem and the extra options the 5way allows. Like the Custom 24, its a slimmer body too. If you are moving from most LP to most PRS the Custom 24 would be the most PRS with the 594 most LP. The McCarty, as you know, is the next step after the 594 and the Custom 22 would fall between the McCarty and Custom 24 - if you can picture that.
If you go for a Custom 22, that would be a modern step up from a McCarty and the Custom 24 would be the next step to the more PRS sound. It really depends more on what you want. The 594 would be a step back in time, being a more vintage sounding instrument to the McCarty with the Custom 22 bringing a more modern flavour. The Custom 24 is more of a PRS flavour to the modern Custom 22 - that's the way the core models are placed.
The 408, 509 and Pauls' are 'Speciality' instruments in that they don't really fit in the same line as the Core. By that, I mean they don't slot in to the chronological (sound from Vintage to Modern) and style (from LP type to PRS) in the same way. These also are instruments that you buy because modifying isn't easy if at all possible - like your 513 for example. That sets these apart from the others above. Paul's is a Signature model too so belongs in that category too like the DGT and Tremonti. The 408 and Paul's are more traditional in their sound (because of the double humbucker and spacing) but with own twist and both have the mini toggle splitting. I have already said the 509 is very similar to the 513 inc the same scale length too so maybe too close to what you have.
Considering your opinion on the SS and HB guitars, really the next 'core' that is most different is the Custom 24. The other guitars are 22 fret models that are similar to your DGT and McCarty with the 594 being more vintage than you have and the Custom 22 more modern. The 408 and Pauls have their own twist on the 22 fret model. It really depends on what you are looking for and 'vibe' too - whether you want more modern or Vintage, more PRS or more of a LP with some 'twist' - at least from the current core line-up with Used bringing some more models. The 509 is a 'twist' on the 513 - slightly different PU's, sound and switching so really it depends on the direction you want to go with adding more PRS to your collection. That 'direction' could be a different Strat type or Hollowbody type adding in a Piezo too of course. Point is, the direction you wish to go from what you have to what you want/need is up to you. For example - want a more vintage sound/feel than the McCarty, the 594 is the guitar, more vintage strat - the SS, more modern LP - Custom 22, more PRS guitar - Custom 24, more LP with a twist but little option to modify - 408 or Pauls, HB with the option to blend in, split to 2 different Amps or use to get a more Acoustic sound - the HBii, want more of an Vintage feel HB with options to split coils - HB594, want something slightly different with independent HB splitting than the 513 - the 509.
You have directions you can go from ALL guitars you own. there are multiple PRS guitars available you can buy new in numerous directions from your starting point, the guitars you own. It really does depend on you and which 'direction' you are looking to add, looking to fill or expand the options you have in some way. In fairness, with the guitars you mentioned in your original post, the most logical is the Strat or HB options - hence I recommended these more but the SS may or may not be too much of a step from your current Strat but only you know if its worth checking out at this point as I know many Strat players that thought they had the 'strat' they wanted until they tried a SS. The HBii isn't quite the same as a 335 so don't make the mistake of thinking that it is or dismiss the Piezo as that can really add something when either blended with the magnetic pups, split off to its own Acoustic amp/PA or used on its own to give you an Electro-Acoustic type option - some great tunes have acoustic opening sounds or an acoustic in a different channel. The ability to add more chime to a magnetic HB pup sound is also very useful. I maybe mentioning these two a bit much but all I am saying is don't dismiss them thinking they are just another Strat or like a 335. Maybe its not the direction you are looking for right now - which I totally understand - but I hope you find this useful in determining which direction you prefer to move in at this time.