Congrats with the site. I've only just had a glance at it, but I think there's a lot you can do. Make it HTTPS first of all, research all keywords that relate to sudoku, write a long article about it and have it on the main page. Make sure you on-page SEO is good and keep doing off-page SEO (getting backlinks). If you want to take SEO a bit more seriously, read through a guide like this to start with: https://backlinko.com/seo-this-year. That's what I did to de-mystify SEO in the beginning.