neither sentence sounds natural. If you are talking prospectively, then "I will end up being a teacher" - or perhaps 'becoming a teacher' is better. If "I am becoming a teacher at the end" means, at the end of your training course/university degree you will qualify as a teacher, then you can say "I will end up with a teacher's qualification', "I will graduate as a qualified teacher". When I finish the course I will be a qualified teacher, etc.

I will end up being a teacher. (Future) More likely. Yours is not impossible, but less likely. "am becoming" suggests something that takes time. "in the end" suggests a single point itn time. so I will be a teacher in the end. <-- more likely. I am becoming a good teacher, slowly, after years of trying. I will be a good teacher in the end.

No difference in meaning. Just "I'll end up" or "I ended up" makes more sense to me. The difference is only structural: end up is a verb, in the end is a time marker.