Everything is hackable and there are too many factors on how it will be hacked. Trustwallet is just an app and for it to be safe will always play on the owner to be honest, let's say 80% of hacks on any wallets can be traced onto the owner's hand, may it because of carelessness or someone really wanted to exploit you but still your responsibility if you let them know especially if you don't know how to safeguard your wallets or your identity.
The main factor because carelessness is the cause of wallet hacking, if we save the mnemonic phrase on an external device and protect the PC from viruses, then it is possible that no one can hack the wallet from the trustwallet or metamask. I have been using metamask and trustwallet for several years and there is no hacking history even though I have kept assets in the wallet till now.
You can still be hacked if you connect your wallet on some shady/unfamiliar websites or a phished site so it's always important to check the url that you are visiting and it would be better if you can just bookmark the sites that you are frequently visiting so that you are sure that it was the real one.
Having a good antivirus on our pc or laptop is already enough because it can detect malware which can stole our keys if we save it on the same device but what you are doing of storing your keys outside can give you an extra layer of protection. You just want to make sure that you never connect that device online. This can act as a cold storage.