How are you talking about SEO and small prices at the same time? Since when did SEO apply to little hobby sites? If you really think SEO is just the on page part generated by something like a WP plugin, you’re so far off.
By the way, for the love of everything that’s holly, stop trying to learn SEO if you don’t have anything to invest or even real competition. You’ll just waste your time. This is not a hobby. You don’t really learn anything until you have something to lose.
SEO hasn’t been about just your website for a while now. Just because you have a website talking about aquariums and you have traffic from Google from aficionados, that is not SEO. That is search engine, without the optimization part.
And regarding WP and the other pre-made tools and websites, they’re not good at SEO, Google is just good at parsing them, because it’s full of them. Most of the time what they call SEO is on page SEO which for the most part means actually structuring the code so that the engines understands what the page is about. Thinking that that’s SEO is like thinking that if you copy paste the source code of an NY Times article on your site, Google should divide the traffic equally between the sources.
SEO is about link building and authority. You can’t do that with just your site. And you can’t build anything note worthy on a $300 a year budget. Even if you discover the most fresh and empty micro niche, in a few months it will be saturated because other people will see that it works for you, and then you’d have to start spending money to stay afloat.
And enough with the naivety and thinking that everyone’s doing himself and with in page SEO we can all survive equally good. SEO a lot of times means taking down other competitors by employing bad SEO on them, by reporting them to Google for idiotic reasons, by sending them what Google considers spam traffic, etc.
I’m not saying that you should do this, I’m saying that it happens and you will be hit with this. Try to stay out of that with switching from Weebly (what a puerile name BTW, they might as well call it Sandbox and style the logo with cartoonish letters) to WP.