Maybe へた would be better than じょうずじゃない but doesn't へた imply that you are awful at it?
Haven't learned ほとんど but is probably better than すこし. Totally forgot about あまり, makes a lot more sense..
As for the last one, Short forms is the 'informal' conjugations よみません > よまない , たべます > たべる.. so I was trying to write all my sentences into short/informal speech (with particles though).
Thanks for your help!
Strictly speaking, へた means you are unskilful, clumsy, so yeah kind of awful... but in this context, the idea is to put yourself down to show you are humble, but still, you keep trying. I would say about myself 日本語がまだまだですが ("not good enough by far"), which is a lot lower than my current level.
For short term, I thought about another meaning. 省略語 is about shortening sentences and words, which is extremely important in Japan. You don't say "go to work", "inside the car", "nuclear power plant" there are are words for it (出勤、車内、原発). For me that's what makes Japanese so difficult. You can't really ouptut based on logical thinking, or get used, you really need to learn a massive amount of vocabulary.
So, とおもいます is a little bit formal and inappropriate for your meaning. Still it's a very usual pattern, it's similar to saying "I'd like" rather than "I want", it's softening. In most languages I know of, especially Japanese, politness can be a mark of distance or reserve, but also a mark of respect (which is also some other sort of distance), including the respect of other thinking, that's why it's softening.
Anyway, you mean informal. Strange thing again, たべる is the dictionary form, that does not include politness. That's why it's casual, but also why newspapers are written in dictionary form. Because there are not adressed to someone, it would not make sense to use a polite form.
So, I'd say カジュアルで生きた日本語で話なすようになりたいな (かじゅあるでいきたにほんごではなすようになりたいな)Prepare to make a strong impact with this sentence lol 生きた日本語 means natural Japanese. ようになる means to become able to. Perhaps it's a little bit formal, but I can't think about a better way to say it.