Both are equally common. It's analogous to saying "thank you" or "thanks".

Usually 谢谢 is used. 谢谢你, while colloquial, is actually grammatically incorrect, as there is a redundant word. One doesn't show more gratitude than the other; they mean about the same thing.
Other examples of saying thank you or showing gratitude include 多谢、谢谢你啦 which is colloquial as well, and the formal 非常感谢e.g. 您对我们的支持 i.e. Thank you very/so much for your help towards us (or any acceptable equivalent).

You can use both because there is little difference,谢谢你 is just used to show more grattitude as you are implying it directly to the person you are speaking to

From what ive heard from my Chinese teacher, 谢谢你 is never used and the 你 is redundant

《谢谢你》 typically is more formal and shows more gratitude than the condensed 《谢谢》. It is similar to how "thank you" vs. "thanks" is.

《拜拜啦》 (bài bài la) is used more frequently in Taiwan, and sounds just like bye-bye. I would argue that while 《再见》 is the standard, it can come off as rather formal or vanilla. There are other ways to say goodbye: 明天见 (míngtiān jiàn, "see you tomorrow", use only if you're actually meeting again); 再联系 (zài liánxì, "let's stay in touch", used to emphasize staying in contact even though you might not be physically); and even 有空再聊 (yǒu kòng zài liáo, "When’re you’re free, let’s chat again", very informal). Have fun!

It's very low/deep, especially compared to the words preceding and following.

I see good reviews for Hello Chinese, although I have never tried that before. The pencil and paper method is the most effective for learning and remembering the characters themselves.
Just a tip, there aren't spaces between each 'phrase' or 词 in Chinese, “谢谢你们。” would do. On that note, I hope this has been helpful and if so, 不用客气; otherwise, keep the questions coming and perhaps someone else will reply with a better, clearer answer.