SEO expert since 2006. Founder https://www.skyinternetmarketing.nl/. Ranking #1 for SEO specialist and SEO expert since 2013 (https://www.skyinternetmarketing.nl/seo-expert/)
After more than 8 years of running my own digital agency https://www.skyinternetmarketing.nl I would say to myself: focus on what you do best. That means offer services you know you can do really well. Don't buy software you might need someday of start a new service. Let other people help you do the bookkeeping, technical stuff, etc. It's good to know something about everything, but be great at just a few things. People will always ask you more if things go well, but don't fall into that trap. Just respectfully say no and help them out by pointing to other people you trust can do that job for them. It saves you stress and other people, including the client, are happy with you.
There are a few great websites about SEO out there. I reccommend: https://moz.com/learn/seo and https://moz.com/blog/category/whiteboard-friday , https://ahrefs.com/blog/ and https://backlinko.com/blog to start with. That should give you more than enough material to study :) Also, https://searchengineland.com/ for frequent updates about what's new in SEO.