got the solution now. Sharing for anyone who might get benefit of the approach. Thanks

{ "rules": { ".read": false, // disables general read access ".write": false, // disables general write access

"Users": {
  ".read": "auth != null", // Users can read all profiles
  
  "$userUid": {
    ".write": "auth != null && auth.uid === $userUid", // Users can write only their own data
    
    ".validate": "newData.child('userUid').val() === auth.uid && 
          
          (!newData.child('userID').exists() || newData.child('userID').isNumber()) &&
          (!newData.child('userName').exists() || 
           (newData.child('userName').isString() && newData.child('userName').val().length > 0)) &&
          (!newData.child('userFCMToken').exists() || 
           (newData.child('userFCMToken').isString() && newData.child('userFCMToken').val().length > 0)) &&
          (!newData.child('userPhoneLanguage').exists() || newData.child('userPhoneLanguage').isString()) &&

          (!newData.child('userProfileImage').exists() || 
           (newData.child('userProfileImage').isString() && 
            newData.child('userProfileImage').val().matches(/^https:\\/\\/xxxx\\//))) &&

          (!newData.child('userBusiness1Id').exists() || newData.child('userBusiness1Id').isNumber()) &&
          (!newData.child('userBusiness1Name').exists() || 
           (newData.child('userBusiness1Name').isString() && newData.child('userBusiness1Name').val().length > 0)) &&
          (!newData.child('userBusiness1ProfileImage').exists() || 
           (newData.child('userBusiness1ProfileImage').isString() && 
            newData.child('userBusiness1ProfileImage').val().matches(/^https:\\/\\/xxx\\//))) &&

          (!newData.child('userBusiness2Id').exists() || newData.child('userBusiness2Id').isNumber()) &&
          (!newData.child('userBusiness2Name').exists() || 
           (newData.child('userBusiness2Name').isString() && newData.child('userBusiness2Name').val().length > 0)) &&
          (!newData.child('userBusiness2ProfileImage').exists() || 
           (newData.child('userBusiness2ProfileImage').isString() && 
            newData.child('userBusiness2ProfileImage').val().matches(/^https:\\/\\/xxxx\\//)))"
  }
},
    "User-Messages": {
  "$userId": {
    ".read": "auth != null && (
      auth.uid === $userId || 
      auth.uid + '_' + root.child('Users').child(auth.uid).child('userBusiness1Id').val() === $userId ||
      auth.uid + '_' + root.child('Users').child(auth.uid).child('userBusiness2Id').val() === $userId
    )",
    "$recipientId": {
      ".write": "auth != null && (
        auth.uid === $userId || // Sender can write to their own path
        auth.uid === $recipientId || // Sender can write to the recipient's path
        $recipientId === auth.uid + '_' + root.child('Users').child(auth.uid).child('userBusiness1Id').val() ||  // When sender is business1, sender can write to user's path
        $recipientId === auth.uid + '_' + root.child('Users').child(auth.uid).child('userBusiness2Id').val() ||  // When sender is business2, sender can write to user's path
        auth.uid + '_' + root.child('Users').child(auth.uid).child('userBusiness1Id').val() === $userId || // Business1 sender
        auth.uid + '_' + root.child('Users').child(auth.uid).child('userBusiness2Id').val() === $userId // Business2 sender
      )",
    "$messageId": {
        ".validate": "newData.child('chatId').exists() && newData.child('chatId').isString() && newData.child('chatId').val().length > 0 &&
                      newData.child('fromId').exists() && newData.child('fromId').isString() && newData.child('fromId').val().length > 0 &&
                      newData.child('toId').exists() && newData.child('toId').isString() && newData.child('toId').val().length > 0 &&
                      newData.child('timestamp').exists() && newData.child('timestamp').isNumber() && newData.child('timestamp').val() > 0 &&
                      newData.child('text').exists() && newData.child('text').isString() && 
                      newData.child('text').val().length > 0 &&
                      newData.child('messageIsRead').exists() && newData.child('messageIsRead').isBoolean() &&
                      (!newData.child('membershipRequest').exists() || (newData.child('membershipRequest').isString()))"
      }
    }
  }
},