What are some of the most reputable tools for doing this? Something that many users have used for years without issues.
Note: I don't want to rely on only one of those two. I want to combine both.
Some software and hardware wallets have already implemented the feature you are asking for. The way it works is you add several bytes of your entropy by flipping a coin or rolling dice and, in case you didn't provide 256 bits of entropy manually, then an in-built cryptographically secure pseudorandom number generator generates the rest bytes.
Check this, for example:
https://bluewallet.io/docs/manual-entropy/Bluewallet is an open-source non-custodial wallet, so everything can be found on their Github page:
https://github.com/BlueWallet/BlueWallet#readmePlease note that unless you're a skilled programmer capable of verifying everything yourself, including the source code behind the seed generation mechanism, you will always have to rely on and trust others when interacting with software. If you don't want to trust, then generate everything manually.