Any 256-bit number between 0x1 and 0xFFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFE BAAE DCE6 AF48 A03B BFD2 5E8C D036 4141 is a valid private key. Assuming your random number is in that range (and it's extremely likely that is the case), it should be just as fine of a private key as any other number. In fact, you shouldn't even bother with checking if the address is already known because:
- Probability tells us it isn't used
- Its existence on a chain explorer is not proof that nobody else has generated that key
- By checking, you may be telegraphing that you own that key
Be more concerned about how your key is generated, rather than is a particular number is vulnerable to some unknown attack. Make sure your dice rolls are done in such a way that every byte is just as likely as every other byte. I've used a D8 in conjunction with a coin flip in order to generate 4 bits at a time. If the coin is heads, I use the D8 at face value. If tails, I add 8. I count 16 (tails + 8) as 0. If you work it out, you'll see that this generates a perfectly random number between 0 and 15, or one hexit. Do this 64 times, and you have a 256-bit key.