Using just dice for achieving true randomness would be more than enough if done properly, and adding anything more could actually reduce random result.
There is no reason to overcomplicate things but I guess you could use Geiger counter that is measuring radioactive decay, one of the most random events in nature.
Some hardware wallets are already combining multiple sources to achieve better randomness but there are some flaws in this method.
Only Keystone, Passport and Coldcard have verifiable seed generation.