Amazon reviews website - growth?

Hello guys. I wanted to get your advice.
I purchased a website 9 months ago.
it's a fitness products reviews Amazon affiliate website, making around $1,500 passively every month only from amazon. Visitors coming only from google. They are actively looking for home gym equipment reviews - which means they are in "buying mood", and I really want to try leverage that more.

I bearly even added any new content or changed anything for over 8 months ago because lack of time.
So, recently I saw a slight decrease in the revenue and in the in traffic.
I know I should add more reviews for newer products, collect more emails, do more SEO (which link building kind'a hard only if I have reviews in the site - any suggestions about that?)
We're also doing some massive redesign. Anything I should add in the new design?
But there is something specific I can do to leverage the fact that my visitors are in buying mood? Anything specific I can market for them by email? I don't have a product, so it all needs to be affiliates.
I would really appreciate your ideas. Please PM me if you want to see the link
Thank you!