Ironically, GrapheneOS is pretty much that (minus the hardware) unless you want to get an iPhone or Windows phone. You’ll have to have Play Services to play the games you want to play; may as well use the only sandboxed version afaik. The reason the Pixel line was chosen is because it’s essentially what iPhone is for Apple… There will be no vendor add-ons or modifications, just stock android which can be modified into things like GrapheneOS. But it’s more for security and privacy than it sounds like you need. Just keep in mind whatever you choose, cracked apps have a much higher chance of containing malware unless you learn to crack them yourself, but then you’d have no time to game.

https://grapheneos.org/usage#sandboxed-play-services

Since the Play services apps are simply regular apps on GrapheneOS, you install them within a specific user or work profile and they’re only available within that profile. Only apps within the same profile can use it and they need to explicitly choose to use it. It works the same way as any other app and has no special capabilities. As with any other app, it can’t access data of other apps and requires explicit user consent to gain access to profile data or the standard permissions. Apps within the same profile can communicate with mutual consent and it’s no different for sandboxed Play services.

ETA: It looks like there is a modified open source Play Services client called MicroG, which is basically a rewrite of Play Services that allows things to work, but things requiring payments or payments via NFC etc may not work.

I think you just want to play games without play services, not actually learn apk reverse engineering (which is a very complex skill), so MicroG is probably what you want. https://microg.org/