So SEO is probably an important aspect of your passive website income. I have a site that's live now but it is totally javascript (clojurescript on the front-end) and there is absolutely no SEO yet because there are no persistent URLs or server-side rendering yet. Your message makes me throw this back up top on priorities. Make magic and make sure people can find it!
Obviously it depends on the type of business you are starting but even on mobile people use search so SEO can be a huge source of traffic
Any advice? Current revenue is 100% AdSense.
I especially resonate with the whole SEO traffic for building websites. I had a niche website a few years ago that made around $40 a month passively without touching it for a year.
It can be a good source of income and not a ton of work involved if you're able to outsource the content generation.
Build a website. Because of SEO traffic this can be pretty passive. I have a few websites that are making income that I stopped working on years ago.
Invest. Investing is a great way for passive income you can look for dividend stocks, REITs or just plain real estate. Take a look at some options here https://senzu.io/investing/compare
For actively making money you can try to do contract work in your free time