How do I prepare my app for the 64 bit version for React Native?

you are almost there remove ndk,

ndk {
    abiFilters 'armeabi-v7a' , 'x86'
} // these are not necessary it won't affect anything

then add in abi -> include

abi{
   ...
   include "x86", "x86_64", "armeabi-v7a", "arm64-v8a"
}

last add versionCodes,

def versionCodes = ["x86":1,"x86_64":2,"armeabi-v7a":3,"arm64-v8a":4]