Here's the animation (post #3) that explains it. Genius work whomever did it:
https://forums.prsguitars.com/threads/custom-24-s2-vs-custom-22-s2.9152/
Which perfectly illustrates the point I was making in that its not the Neck pick-up that moves but the Bridge and Bridge Pick-up due to the longer neck. This does make it a bit easier to play fret 22 for example as its slightly further away from the body and the 'feel' is slightly different because of the bridge position and slightly longer neck - most notable I think when standing and using a Guitar strap. Maybe some won't really notice and make the adjustment naturally but I was just trying to give people the correct information.
If people are used to a Custom 22 and find fret 22 a little bit of a stretch because of where it sits in relation to the body, the heel etc, adding to frets to the end of the fretboard and moving the neck back may make people think those frets are 'pointless' because they can just about reach fret 22 let alone frets 23 and 24. A lot of guitarists also play with their hand on the bridge and just in front for muting with their picking hand but with a Custom 24, you have to move your hand a bit further to the neck and when its hanging on a strap, that feels more noticeably different - even if its only a small change.
As for the 'harmonic' which would be above the neck Pick-up for an open string on the Custom 22, it doesn't make a difference in my opinion. In general playing, how often do you ever use the 24th fret Harmonic? Even if you do, does it really make a massive difference - one that can be quantified? One that cannot be attributed to just the difference in tone that the position of the Neck Pick-Up has? If you play a Barre Chord on the 5th fret for example, the harmonic changes position anyway and would favour the Custom 24 over the Custom 22 if it made any 'significant' difference. Any difference tonally is down to the position of the neck relative to the bridge where the string 'starts'.
I think the Custom 22 is a bit more like a modern update to the Les Paul. It has a few more tonal options because of the 5-way and the Trem too of course. The Custom 24 is more of its own thing and the Guitar that launched PRS into the world. Its that classic in between the two 'big' icons that were PRS's competition in 85 and as such, its much more its own thing. If that sits midway, the Custom 22 is a bit more towards the Les Paul. If you want the 'ultimate' Custom 22, the PRS Special semi-hollow could well be the answer and the NF in the middle adds so much more than a LP offers - even if 3 of the 12 Pick-Up combinations are similar to the LP's 3 options.
Based on the fact the OP has a Les Paul and LP Jr, I think I would recommend either looking at the Cu24 (particularly the Cu24-08 or the Cu P24 the Piezo offers something 'extra' the OP doesn't have) or the PRS Special 22 Semi Hollow - its the most versatile PRS (excluding PS MEv) but also being a Limited Edition, something you ought to pick up sooner rather than later because they could be gone soon. I would recommend the PRS Special over any of the other Cu22. The other option I would recommend is the Hollowbody 2 if you fancied adding a Hollowbody to the collection. Its similar to the 594 - same PU's but also adds a Piezo if you wanted to add a bit more of an acoustic chime to your collection but haven't appeared to want a 'hollowbody', this is perhaps a more of left field recommendation.
I think the McCarty and McCarty 594 are certainly closer again to the Les Paul with the 594 being much closer to the Vintage LP than any PRS. It could replace the R7 or be too similar that you may not get the use out of it. The Signature series could offer something - the DGT (maybe too close to the R7 but has a Trem), The Tremonti (a more Rock/metal LP), Paul's (a bit like a LP maybe more P90ish on sound from the smaller humbuckers but great SC split) and of course the Santana too which could be a great alternative.
Based on what you have though, I think the Cu24-08, P24 or Special 22 would be a better fit to give you something different and add more to your tonal palette than any other core PRS. At the end of the day though, the OP ought to try the guitars themselves first and decide. My recommendations could be used to help the OP decide which guitars to look at, to try first. All that's left to say is good luck, enjoy your search and look forward to seeing your NGD post in the future...