You haven't done anything wrong yet, when you have new cold card just create entirely new seeds and move the funds to it and that's it the funds again went back to cold storage.

You can't be 100% sure that any device is free from any kind of malware even the most advanced machines with all the necessary things from government agencies can't stop the remote attack from hackers so think about security of your device. For now you can use the wallet meanwhile not connecting to internet can be a better than always connected to internet.