In case an affiliate program doesn't allow you to promote competitors, go into complementing industries. For example, if you're promoting a weight loss training program to your audience, also start promoting fitness products from Amazon since that's a natural choice for your audience.
There is no specific ideal number of followers for affiliate marketing, however, a good number of 5,000 to 15,000 followers is enough to become an affiliate marketer. Also, your engagement rate should be at least 100 likes per post. Don’t forget, marketers also track your performance and organic engagement!

Some of your keywords are probably already targeted by very powerful websites. So, hard as you try, you won't be able to rank in good enough positions to attract visitors. To check which keywords are too competitive, go to the Keyword Sandbox in Rank Tracker and check Keyword Difficulty for all discovered keywords as a group or for each individual keyword:
Once you have a somewhat established website and have gained some credibility within your chosen niche, you can think about reaching out to other website owners and offer guest posting services.