First of all - don't use a web source. As soon as you enter your entropy in to a website, then you have negated the entire point of creating it manually. Whatever you use to convert binary in to hex, make sure it is done offline (you can also do it manually with a simple lookup table such as this one: https://i.imgur.com/BjpomPf.jpg).
Second of all - don't memorize it. Back it up written down on paper. If you try to memorize something as complicated as a random private key, you will almost certainly forget some portion of it and lose access to your coins.
Lastly, I always think it is better to work with seed phrases rather than individual private keys. They are easier to use, harder to make a mistake with, allow you to back up an entire wallet rather than just a single address, and avoid issues with coins being sent to change addresses you cannot access. It is also very easy to use your own entropy to generate a seed phrase by selecting the box "Show entropy details" on the Ian Coleman page.
I agree with everything you’ve said and can add even more to that.He could obtain the SEED phrase from 256-bit-long random number resulted from coin flipping. The procedure is straightforward and can be manually done without reference to any online service. Informative description of such procedure can be found over there or DYOR to look for other source of pertaining info.