I really don't have experience with those offline wallets but if somehow my PC get burned or something & I can't fix .. does it mean I will lose all my bitcoins unless I took a backup ? if yes how to take a backup
Shutdown bitcoin core. Then find wallet.dat, and copy it to a usb drive or two, store those at the office or another location you see fit. Make sure you've sett a passphrase on the wallet. Also realize that once you do transactions, change might be sent to new addresses in the wallet, requiring you to do a new backup.