Native speaker here. I'd say 'dankjewel' (or 'dank je wel') is informal, 'dankuwel' is very formal and 'bedankt' is somewhere in the middle. Context and accent will play a part in which one people choose. You could use 'bedankt' anywhere without being weird or rude.
Dag can mean hello or goodbye. Literally, it just means day. Doei is goodbye, but it is less formal than dag. Tot ziens is only goodbye. Literally it means something like "See you!" I think it is just as formal as dag, but I'm not certain of that.
Ik gave the right translation, except for the capital latter. I started with a t instead of T. But the answer was wrong according to the computer. This should be corrected. Thanks, bye is the same as thanks, bye