Okay, so I didn’t see this horse in motion with a saddle on and you riding, and I am only judging based off the pictures you put here (which can be totally different than in person), so take this with a grain of salt, but I’m going to go on a slight tangent here…
In my humble opinion, shimming the front of your saddle doesn’t make sense unless its for a wither clearance reason because he does have a more uphill build with a larger shoulder. However, shimming the saddle for wither clearance might create more pressures along the front of the saddle, as he does have a larger shoulder, and it is likely going to throw your balance off, to where you might want a back shim as well (or a 305 panel).
Again, this is purely based off two photos and to be fair - the horse is not square in photo one and I don’t know the conditions of photo two. Also, your horse might really round up his back under saddle and you may not need anything else other than some wither clearance. I will agree: at 4 your horse is going to change, so the perfect fit now might not be the perfect fit later. However, on the flip, maybe he doesn’t change much at all and you have a great fit for a long time (that just happens much less).
I guess my questions would be:
When you ride in the PA ST RT - did you feel like it had enough wither clearance without any shims, and did you feel like it fit the horse shoulder area well (no pinching, no sliding, etc)? Did you notice any difference during and after you flatted?
IMHO, I’d try and avoid any shimmable half pads, if possible, upfront if none of those things were a concern. A shim in an area where it might bridge while muscle is developing (mid back, if your horse doesn’t round out under saddle) might be a good option, and depending how he rides, you might want a little shim in the back (or a 305). I’ve added 305s in saddles just because my client liked that balance a little better. Honestly, at least back in my day, a 305 didn’t cause too much trouble upfront unless the horse was real withery.
Here is my general though ton half pads/thick pads (I’m looking at you western people) in General. You don’t add thick socks on top of your regular socks to help relieve pressure in your shoes that are a little tight, because it creates more pressure, so instead you generally try a different brand of shoes that fits better. The same standard should be held to saddles.
TLDR version: A 305 might not be bad on this horse, depending how he moves and how you like your saddle balanced, but I don’t know that you actually need to shim the front of the saddle. 5mm is really only ~.20 of an inch, so less than half an inch, which isn’t a whole lot on most horses. However, I wasn’t boots on the ground, so I couldn’t really be sure why she wants you to shim the front or how the ST saddle actually sit/rode. It could be worth a try if the person selling will let you trial it!