1. I'd go to the GSC and use a filter to see pages that lost clicks\views with prev period
2. Compare all those pages with SERP, observing by yourself
3. GAP feature in Ahrefs
It sounds a little bit crazy, but I'd recommend POGO advertise and help rise licensed casinos. Otherwise, offshore casinos will win. Among 109 million people a lot of players. Besides, China is not so far.
I'm trying not to work with keywords and casinos with the word "free". Percent to real players from free is quite small, but Keyword Difficulty is high)
After all, I'd say that the best WP theme is your own. up to 1k for nice unique design and up to 1k for dev and extra feature. If not, then Poka, Mercury, Vegas.