I think at a certain point, it’s really up to hands-on experience with a lot of site diversity:
– Different site sizes
– Local, National, International
– Different industries
– Different CMS’s
Working on unique sites will bring up unique problems and therefore more opportunities to learn outside of basic theory.
There are a few advanced concepts that might be outside of core SEO, but learning about them will make you better at it imo:
– General knowledge on information retrieval
– Studying Google patents
– Code (HTML, CSS, JS, Python)
– Page speed and UX
– Conversion rate optimization
And lastly, concepts and theory are great, but learning how to function inside of a business where there are multiple channels and stakeholders, but limited resources, will take you from good to great as an SEO.
The skill of storytelling, demonstrating impact, and knowing how businesses run successfully is so important.
SEO knowledge is fun for us, but no CEO in the world cares about obsessing over schema.org and anchor text optimization.