So to the powers that be please add this to the bucket of wish list items.
Not going to happen, imho, as it's a niche request.
You'd be better off requesting a mod[ification] to be created, using the
Mod Request Board but be prepared (or advise your forum admin to be prepared) to pay for this service

It wouldn't take much effort to create an Admin function that could be clicked to invoke, or even run as a scheduled task, that produces a monthly report in csv format.
This, for example, would output the board names plus number of messages created between that same range of dates:
select b.name, count(*) from smf_messages as m left join smf_boards as b on m.id_board = b.id_board WHERE poster_time BETWEEN UNIX_TIMESTAMP('2020-01-01') AND UNIX_TIMESTAMP('2020-01-31')
group by b.id_board;