I shoot the 5' one-piece with the 2' extension. The Pro wasn't out when I bought mine, but I use it as a walking stick all the time, and I haven't had any trouble at all, and I weigh around 250 pounds.
You need to practice with the blowgun set up the way you intend to shoot it. If you practice without the extension, and get good, then put the extension on, it's a different blowgun now, and you have to start all over. The extension gives you at least 1/3 more power, and range, maybe more. I can hit my archery target (somewhere) as far away as 45 yards with the extension. Not good enough to hunt at that range, but all darts on the target somewhere.
If you are breaking darts, you are too close. I practice at 15-30 yards. If you are going to shoot much closer than that, you need to not use the extension. Even at 10 yards, the 5-foot is going probably be too powerful. At 15 yards, without the extension, I can send a broadhead completely through my archery target, and out the other side. I can completely bury a 1-foot bamboo dart in the target at 15 yards. The CS .625 is not a toy, like other blowguns. It is made to take game with, and do it well. You may want to get a .40 or .50 cal for target shooting. I have both, and I use the .40 cal for shooting at close range targets. I also break it down to 4' for shorter ranges.
It sounds like what you are doing is a lot like trying to shoot a 30-06 at an airgun range. Move farther away, or get a smaller shorter blowgun.