Without correct on-page you're wasting your time.
It's like a porsche without wheels. Useless.
You can fool the algorithm and rank garbage dummy content if you know on-page well.
Google does not understand content anywhere near the level of a human.
Good content is only required for a sustainable site that will attract natural links with time, convert well and attract repeat visitors/growth.
Sure, you might just have an article written and the on-page is naturally decent enough to rank, but what you're doing here if you buy good backlinks is getting your porsche and putting little thin bike tyres on it. Sure it'll drive and it'll get you somewhere, but it's going to get obliterated by a porsche with sports tyres.
I've seen time and time and time and time and time and time again the effect of optimizing the on-page on an existing site that's ranking on page 2-3, or the top of page 1.