Walther has two product lines; the duty oriented guns produced at Ulm, and the consumer oriented guns produced by Umarex (the airgun maker). The PPQ, P99, and I think the Creed are produced in Ulm. The others, like the PK380, P22, etc are produced by Umarex. I agree, the Umarex guns are airsoft level guns. I own a PPQ, and it's every bit the quality of a Glock.
I'm not sure your reasoning on having a replaceable grip frame is sound. In 20 years, will Beretta Pico or Nano grip frames be available? These models generally don't stick around that long. If these frames are weak, then all of the grip frames made during the production run will have been purchased by users with broken frames. If not, then all of the frames available in 20 years will be 20 years old. They'll be more likely to break. I'm afraid the era of the polymer frame means that guns are no longer heirloom pieces. They're more like cars, with a certain, designed lifespan.
I do think you're taking the right steps by doing the research necessary to upgrade your pistol. However, if you can't find a pistol with demonstrably better ergos for you, I think you should be hesitant to change pistols. There's always the risk that the pistol you buy will be a lemon, and based on your circumstances, that would be disastrous. Now, quality brands are very unlikely to put out a lemon, but the possibility is always there.
With respect to the .380, it's your gun and circumstances; you know best. The market in the US has moved away from small .380s to small 9mms, because the power differential is substantial for a very little bit more in size. It's very possible to deep conceal a G43 or Shield, even if it is tougher than smaller pistol. The key driver for me is that the G43 and Shield are well established pistols, with very high quality. The ones you are looking at are not as well established. Does this mean you'll get a lemon, or have parts breakage down the road? Maybe, or maybe not. Do you feel lucky, punk?