I learned SEO by trying out a bunch of ideas and found lots of tools along the way. I'm still using the following ones on a regular basis!
Keyword Research
KeywordsEverywhere.com: This chrome extension gives you info about your search like search volume, trends, and related terms. I love it because it's really simple and useful. It's not free, but you get 100k credits for $1,25/month!
Search Response.io: Great for searching through the dataset of 150M+ People Also Ask (PAA) questions collected from Google across 200M keywords. You can do topic research, and see all PAA questions answered by specific sites. Very useful for writing SEO blog posts. The free version is enough for solo makers!
Writing
Koala.sh: Favorite AI content writing tool so far, you have so many criteria to tweak like keywords you're targeting, URLs to link to, getting real-time search results or adding sources. You get 5000 words for free, and then 15 000 words/month for $9.
SEOReviewTools.com: Make sure your blog posts are optimized for SEO, with a main keyword :)
Hemingwayapp.com: to write a good plain English!
Backlinks
Programmatic SEO Pages (want to try)
Monitoring:
Google Search Console
Ahrefs now, but started with Katlinks.io (cheaper for solo makers)
Do you have other favorite SEO tools you use? I'd love to add them to thesolomaker.com/stack.