If you have bitwig, you really don't need much else. Other things can be nice to have, but focusing on new gear/software is a never ending rabbit hole that has a near-complete reduction in productivity at the bottom. Bitwig's included synths, samplers, and effects are WILDLY powerful, and you can do basically anything you can do in any other synth, with few exceptions.
I would focus on trying to make specific things. Like, try to recreate a section of a track you like. If you can't see an obvious way to do a specific part in bitwig, look up a tutorial, or download a sample. That way you'll focus on doing stuff you like, and on learning how to do it better.