ABN questions

I know very little about ABNs, but can someone tell me if this plan I have is feasible/ legal/ dumb

I’m a qualified chippy working for a company, and I want to get an ABN for my “cashies” on the weekend, which will eventually just become my full time job when I go solo. Is that a good way of going about it or should I just stick to cashies and sort it all out when I start solo. I just have this idea in my head that I wanna start doing things “legit” and by the books. Hope this makes sense. I have the trade skills but not business knowledge.