I know that both are important but before I learned about SEO I thought that off-page was more important than on-page. But now I realized that on-page is very important. Off-page will be built over time if you do the on-page correct. You just need to be patient with off-page.
Depends on your DA. Low DA site? Off-page is crucial. I've been testing "parasite SEO" - leaving helpful comments on high-DA platforms (Reddit, Quora) that Google indexes. Way faster than building backlinks to a new site. High DA site? On-page optimization matters more for conversion.
I used to think external content was everything too, but internal content is really the foundation. If your site is strong “inside,” external efforts gradually start paying off. The key is not to give up and to give backlinks and guest posts some time to work.
Both have value combined, but if your on page isn't working then nothing you do off page even matters.
I just watch a video a few days ago saying that off page seo matter less then 20% this days, and I for one can attest to that, I have like a 15 year old website I'm and I'm top on backlinks but like a few years ago I had a programmer redo my site, that had nothing to do with seo and from first place on some important terms I want to 4-5 page. I bet if I redo the website myself now I will be on top again, unfortunelly it's a very complex site and I don't feel like doing yet, but who knows, maybe soon !