A prime number with a given number of digits (except $1$) has the maximum possible digitsum (in base $10$) , if it has only digits nine except one digit which is $8$.
A large (probable) prime of this kind is , according to OEIS , $$10^{2\cdot 188484}-10^{188484}-1$$ Is it the largest known such prime number ?
There is website with Near-repdigit-related (probable) prime numbers. It's number 82 in the list there: https://stdkmd.net/nrr/prime/#list_size