There are dozens of methods for key generation (even that doesn't implies software like with dices and so on).
I know that these are not methods for everyone but meanwhile it's something boring that people would just rely in a third part solution WHEN they have already EVERYTHING to be totally indipendent.
Regarding key generation. I used this:
https://github.com/trezor/python-mnemonicon a live USB in air plane mode after installing the Python package.
Indeed this package rely on secrets which CSPRNG and it seems that Trezor use this to generate the mnemonic
on their devices. Moreover, since a computer should have more entropy than a Trezor, i though, but i may be
wrong, that this method is one of the most secure.
That being said, i'm aware of malicious PyPI packages:
https://www.scworld.com/brief/malicious-pypi-packages-set-sights-on-crypto-wallet-recovery-passwordsbut i think i'm safe with the one i used.