I have a G42 for occasional pocket carry and have tried a G43 owned by a friend.
For me, the G42 works very well for pocket carry. Yes, it is slightly bigger than something like a Ruger LCP, but primarily in the length of the barrel and slide. If you have the pinky extensions on the LCP magazines, like I had to have on mine to grip it even moderately well, the length of the butt on the G42 is practically the same as the LCP. The G42 is only slightly wider and is somewhat heavier. By judiciously choosing pants for the size of the pockets (which I do anyway because I carry a lot in my pockets), the G42 can be pocket carried very well. The fact that it shoots so well is just icing on the cake. I REALLY like my G42 and have to consciously make myself carry it over my 9mm Shield sometimes, even if I can carry IWB. Bottom line, for me the G42 is at the upper end of what is practical for pocket carry, but it does work well with careful choices in pants and holster.
As for .380 over 9mm, that is purely a personal decision. I have watched MANY, MANY videos and read many reports about .380 ammo tests. While .380 isn't as good of a self defense choice as 9mm, there are some good loads that will effectively meet the FBI guidelines for penetration and expansion. Not many, but there are some. .380 doesn't have enough power to simultaneously expand well and penetrate well, so the best SD choices tend to penetrate fairly well but expand very modestly. The best choices seem to be any of the various loads using the Hornady XTP bullets, which as said above, expand modestly (but do seem to reliably expand) while penetrating pretty well. This is a necessary tradeoff due to the limited power of the .380 round. For myself, I am comfortable with .380 with GOOD ammo. That is a choice that you have to make for yourself, though. If someone isn't comfortable with .380, I understand completely.
As for mods to the G42, mine is completely stock, so I can't really speak to them. I have been reading up on mods to increase mag capacity and am very interested in the MagGuts springs and followers for the factory mags. They raise mag capacity to 7 rds with the factory mags with no extensions. All the reviews I have read say that they work very well and are as reliable as the factory mags. They are high on my list to buy when I get spare money for gun stuff. IMHO, for the length of the butt, the G42 should have had a 7 rd mag capacity anyway, not 6, but it is what it is. My understanding is that Glock made the mag capacity 6 rds for maximum reliability and durability. I've never seen or used the ETS magazines and have read some reports about reliability problems with them. I'm curious about them and, again when I get the extra cash, I want to get some to try. Extended mags make a lot of sense for spares when carrying, so I want them to work for me. The fact that they don't have a positive overtravel stop worries me a little but I haven't read any reports about problems with overinserting them, so we'll see when the time comes.
As for the G43, I wanted to love it. I almost bought one sight unseen, which I NEVER do. I got to shoot a friend's G43 and am so glad that I didn't buy one. For me, VERY snappy and very unpleasant recoil. The recoil seemed to go right into the knuckle at the base of my thumb and hurt so bad that, even after only 2 mags, my hand was sore for a couple of days. Not bad, but bad enough that I felt it. That's not a slam on the gun, just pointing out that it didn't fit my hand or work for me very well at all, perfectly demonstrating the very subjective nature of handguns and how they fit an individual's hands. Although it isn't a lot bigger than the G42, it was enough bigger that I couldn't effectively pocket carry it. It was just enough bigger than the G42 that I couldn't effectively draw it from my pocket like I can my G42. Since this meant that the G43 would have been a belt carry gun for me, I didn't get one since I carry my 9mm Shield when I carry IWB. If it would have worked for pocket carry, I would have sucked up the pain and gotten one anyway, because 9mm trumps .380, but it didn't, so I didn't get one and don't plan on getting one. If you want a G43, I would STRONGLY suggest trying one before buying one to make sure that it will work for you. The gun seems to work well for most people and my experience seems to be unusual, but I thought it was worth mentioning.
Sorry for the long post. I got carried away again.
Bub