I have done marketing (advertising specifically) for the sports niche.
Test very targetted audiences. If you are targeting basketball teams, football teams, cycling teams etc, test different audiences for those. Show them the ad relevant for just them; such as show cyclist gear to cycling team members. You will find which audience works best for you; all may show very different CTR.
Layer in all the ways you can, such as based on purchase behaviour. You can also test by age, gender and by geo. Age is interesting as you may find two separate audiences working for you - parents buying for kids, and adult sports enthusiasts buying for themselves. You may want to message the two differently.
If you sell spectator sports gear, then that will have different treatment and ads. You will then run ads for that audience if let's say Super Bowl is coming up!
You can test one audience that is made up of sports coaches. Layer this in various ways too, such as by interest in sports for which you sell - coaches interested in cycling, vs coaches interested in basketball. When you make the audiences you will see if you are getting a good enough size per audience.
Also don't wait to build your community. Start building already. Your 'coming soon' landing page could lead people to your fb page, twitter page. The page has 'share' buttons just now, but what would people share? Rather gather these people in a community.
When you start running ads, this may take up most of your time, so put your social media on autopilot with an easy-to-use app called OrangeTwig. Share reviews and testimonials which you will need to build faith in your brand. Show off your products. Do all this with OrangeTwig. Check it out.
All the best!