Going back to SEO

I have not been doing it for a while now except of course the best practices when creating websites. Websites created have decent results on SERPs and lately my older clients asked me to do a more comprehensive SEO. Back in the days (8 years ago) I have a Linkbuilding team, I am not sure I am ready to invest in hiring and training new Linkbuilding team. I have done a bit of research on the latest way of doing SEO and still see my fundamental in on-page SEO still is relevant. I also scoured this Reddit sub (about 50% of it currently). I guess I am asking if going back to SEO, where should I start and concentrate in? Thanks