I think I have been doing some things wrong for years... But I can learn and recover
That sounds like a very good insight. From many of your questions here, I would agree with your concerns. However, there are many others in the same boat around the world and they haven't even realized what they're doing wrong.
The common error I see is relying too much on some kind of formula for "doing SEO". Years ago, that worked. It also gave too much ranking advantage to those who had some kind of workable formula, and Google realized that their earlier way of ranking was not really working the way they wanted.
What is missing in all that? Understanding what SEO really means. It's not a standard set of actions, it's creating an online business in a way that can rank well. Years ago, search engines were more primitive and the first SEOs could reverse engineer the ranking formulas. Then they ended up with a fixed set of steps to meet those ranking formulas. It's not very much that way today, and it's less and less that way all the time.
What you need today is to create a 3-way win: be good for your business, good for visitors and good for Google. And that means you start with a real business and business plan, and not just "a website." And mostly just forget about what you've read that comes from very old sources. That can be hard, because the BS just keeps on spreading, years after it is true.
But if you stay focused on your visitors, your users, your customers - then you can succeed. The idea is not to be some kind of parasite that takes money off the side of a search engine, but to be creating real value in a clear way.
To address your opening question again, yes you certainly can and should use synonyms. But don't make that into just another strategy, another formula. Synonyms are natural in writing or any communication. Use them that way, with perhaps some extra attention on the writing process to make sure it really works and doesn't get stuck in a pattern that works against true communication.