that is the way that many random number generators do it. they take a random input then hash it using SHA256. but remember that the hash is only done to turn the input into a 256 bit (32 byte) output so that it is within the range of valid keys, it doesn't add any randomness since the result is always the same.
and most important of all is the way you are getting that random input. if it is the following as you said in one of your comments:
i can't really say whether this is random or not but to me it doesn't look random enough. if you are so concerned about randomness and don't want to use a wallet that is already doing it then use a dice or a coin flip to generate your random number.