There's no safest way than Manual way. All the private key of your assets must be safe to the Paper. Then laminate them with plastic and save it in your Safety Box. I'm saying this because I see in another perspective. Save them to our device can't fully trusted. A lot of cutting-edge malware program was developed every day, and you don't know, maybe it already been on your device.
Your opinion is true, there is no device or system that is safe even though using antivirus. the safest way is to lock the private key by compressing it using winrar and locking it. Now a lot of malware and ransomware can infect devices very quickly. I myself have also been a victim of ransomware and my personal data cannot be opened, if I want to open it I have to send a bitcoin to get the antidote.