Do not trust any or this software to generate those words truly randomly? No problem. Use bip39 diceware to literally generate your words with dice.
When it comes to not trusting software to convert those words to a keypair deterministically, use NASA dual path tactics.
Use two unrelated software suites to convert those words to keypairs and sign a transaction and ensure the final signed payload output from both unrelated toolchains is identical. Bip39 has been implemented in many programming languages.
Now you can form strong confidence in your entire supply chain from key generation entropy to a final transaction with perhaps a couple hours of effort. If you are moving millions in value and worried about supply chain attacks the extra effort may be worth it.
For most a Trezor or a Ledger used as the instructions that come with it advise will be in fine shape.
That said only people that need to understand tactics this strict are those storing VC levels of value.
For an individual just buy a Trezor or a Ledger and follow the point and click instructions. It is no harder than traditional online banking.
Then you have to trust the hardware storing the data not to fail. But every physical storage medium will fail eventually, so now you need a system by which you create backups and test your backups. And now you have to secure your backups separate in case of fire or technical theft too. It starts to become a part time job very quickly...