i can't defend doing one thing at a time because i've always been involved in parallel projects.
if you learn backend, slash full stack development, you'll pick up minimum viable frontend + design chops along the way. with free resources like Orange and Refactoring UI, you don't need to "be a designer" to build functional, decent looking products.
and hey -- if the only thing between your product being good and being great is design, just hire a kickass designer and implement their work as a final polish. i've done it many times.