You do realize that those machines are not necessarily NixOS right? It is best to separate the management of SSH from NixOS declarative nature since what you would really want to be declarative is ACL rules, not network topology/SSH keys. For example you can use Netbird or Tailscale and their respective SSH feature.
You can change the source of installation to Play Store by using this adb command:
adb shell pm install -i "com.android.vending" -r "path to apk"