No. It needs to be two sentences.

After reading your last comment, I think a good way to express your true meaning would be: “Learning English is simply a hobby that I enjoy.” That emphasizes that learning the language isn’t a necessity but it brings you joy.

No.

Yes, but separate the phrases with punctuation: I learn English as a hobby; that’s my end goal. I'm not sure that it's logical, though; your 'end goal' is the ultimate target to which you aspire, for example, to conduct a simple conversation, or to write business emails. Doing something as a hobby refers to why you are doing it, as opposed, for example, to doing it for your job, or to talk to members of your family.