SEO Tools / Semrush Free Equivalent

Hey,

I'm a total newbie when it comes this so apologies if this comes across confusingly.

One thing I want to experiment with to get some beta customers is SEO and playing about with keywords.

I'd like to do some analysis to understand the keywords that my competitors are using. I'd also like to periodically audit the SEO on my own site, get insights where I can improvement my SEO and then validate if the improvements are actually working.

Any recommendations on the best free keyword tool to use? I really like the look of Semrush but it's pricey for someone bootstrapping.

Chris