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.