If your machine was ever connected to the Internet, consider it compromised when it comes to typing in mnemonics (assuming you're taking the ~security of your bitcoins serious enough).

Turning off WiFi ain't going to save you from anyone more clever than a script kiddie. And I would assume even the malware that script kiddies have access to don't just crash because you disabled your network access. They are just going to wait until you have network access again to exfiltrate data.

Use TailsOS as mentioned by @fishyness here if you really don't want to lose your keys.