Hey there,
No offence, but I'd rather flip a coin 256 times than trust an unverified tool with that. No coder myself, unfortunately, so I can't check whether the tool is not providing me with one of the, say, 10 million keys which are stored in its memory.
Spending extra 25-45 mins is not that much compared to the possible risk of having a private key known to someone else, imho.
None taken, that's obviously the concern I am trying to address here. Its ideal to generate keys completely offline without a computer provided you can use verified secure entropy (as humans are bad source of entropy) and have the know-how. And then rely on an air-gapped hardware wallet to manage everything else

Now, would you go "all-manual" route to generate your keys or you would use a computer at some point in the key generation process? I am trying to offer whatever little computer tool you use in the form of an option, to mix that manual source of entropy into key generation.