The first might not be an exchange's wallet, and instead a part of the
other 20 addresses above may actually be exchange's wallet, or may
belong to the wallet being held offline and then they do not do
anything useful for the network.

Instead you do not feel that would be more serious and fair reward with
an extra, the first 20 addresses that have produced more blocks in the
given period of time?
They actually do something for the network, they make sure that it works.

Extract information of the top 20 staker is not too hard, just count the
blocks in the main-chain generated by address. (with a routine ofc)

Bye Bye