I have some question to ask, what actually is checksum in mnemonic seed phrases, what's the difference between seed phrase with valid checksum and the one with invalid checksum in respect to wallet vulnerability to be easily attack/brute force.
A checksum is used to verify the integrity of data.
In the case of a mnemonic code, the checksum assures that the mnemonic code has been entered correctly (it checks whether it is a valid mnemonic). A checksum is calculated from the data it verifies the integrity from.
In regards to bruteforcing, it practically doesn't really matter.
An attacker has to bruteforce a 128 bit secure (12 word) mnemonic code.