I know it will be a hard thing to fine a single game base casino most especially slot base, but some of the few i have seen have great features and loads of benefits most especially for players with high rank accounts. So am thinking if one can find new casinos that offer such services.
Then forget looking at that. Why put yourself in trouble by looking at those sites wherein there are lots of reputable sites that have a variety of slots games. I don't know what's even the advantage of using a site with only slots games compare to those with a variety of games.
Always prioritize the reputation of the site. They almost have the thing that you are looking for. Instead of just reading here, try those sites in actual for you to experience the real deal and legit self-reviews.
For the benefits or perks, you need to manually check these sites and see what suits for you.