Ghulk will help you with some content but i would wait. 5 stars comes so often now so dont waste resources in lack of other options. You will get good champs fast.
Yeah, I'd say you should still work on your 5*s a bit. At least until you've explored acts 1-5, from there, 5*s and 6*s should be your focus. That was my principle, and it really helped me.
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Also, Gulk and Morningstar are solid, and Magneto might be really good after the buff