Learn English – Beg to differ – Why is there a need to beg for differ

Here's my opinion of your alternatives.

1. "because" is your best choice for clear, correct, unambiguous communication, especially if you want to be understood easily by other non-native speakers.

2. "since" is acceptable, although it makes your statement ambiguous; the second meaning would be that statement A became true at the time that statement B became true. Example: "I study more often since I enrolled in class" conveys that when you enrolled in class, then you began studying more often. Rule of thumb: prefer "since" when your intent is to convey "from the time that".

3. "as" is somewhat acceptable, but in your particular statement structure "as" has more useful meanings of "abstractly analogous" or "synchronously". Example: "I like apples as you like oranges" conveys that my liking is similar to your liking, i.e. equivalently strong/weak/notable/etc. Example: "I make dinner as you set the table" conveys that we do these tasks at the same time in the same place. Rule of thumb: prefer "as" when your intent is to convey "sameness".

4. "for" can be acceptable, although I would never expect to hear it in normal conversation. It would come across as overly-academic, or possibly epic or religious. Example: "We eat well tonight, for tomorrow we go to war." Rule of thumb: prefer "for" when you want to be poetic and inspirational.

I agree that "thank you" is slightly more formal than "thanks". I suspect if I was addressing the Queen I'd say "Thank you, ma'am". I tend to use "thank you" for emphasis, to show that I am really grateful, and it's not just a polite formality. Otherwise there's not a great difference, in my opinion of common usage. (Except for 'giving thanks', but that's a whole other kettle of fish.)