Oddity is the default XP system that gives you a good amount of XP as long as you're constantly exploring new areas, which happens naturally as you progress through the game. You will get reasonable XP no matter what your approach to clearing those areas is, whether you're a stealth character or a combat character.
On Classic, it's very easy to be behind on levels unless you're taking the violent approach to everything, hence I would only recommend Classic for highly effective combat builds that want to hunt higher-level enemies for XP.
I suspect you would be a few levels higher on Oddity, yes. Classic requires a pretty big commitment to killing and the like if you want to keep up. This is the reason why Oddity is default: it's suitable for all builds, and there's no pressure to play the game any specific way. Even without lockpicking/hacking there's still plenty of oddities to be found.
If you're not enjoying your build, I would probably recommend restarting on Oddity, with a slightly more minmaxed build. The thing about minmaxing in this game is that an effective build also tends to be a more fun build. The more you specialize in order to get the most out of a specific approach to the game, the more effective you are, and the more unique your build is. Various feats and equipment from crafting can come together in a very satisfying manner.
In addition to being underleveled, a partial reason why your gun damage is lacking is probably the combination of low perception, and the fact that you're buying guns. Of all builds, guns benefit perhaps the most from crafting, as you utilize 7 intelligence feats like Gun Nut. And even without those feats, crafted guns tend to be stronger.
