if you want to back this file up regularly with something like restic, then you will quickly end up in a world of pain: since new mails are not even appended to the end of the file, each cycle of takeout-then-backup essentially produces a new giant file.
As I'm sure the author is aware, Restic will do hash-based chunking so that similar files can be efficiently be backed up.
How similar are two successive Takeout mboxes?
If the order of messages within an mbox is stable, and new emails are inserted somewhere, the delta update might be tiny.
Even if the order of the mbox's messages are ~random, Restic's delta updates will forego large attachments.
It would be great to see empirical figures here: how large is the incremental backup after after a month's emails. How does that compare for each backup strategy?
The pro of sticking with restic is simplicity, and also avoiding the risk of your tool managing to screw up the data.
This risk isn't so bad if it's a mature tool that canonicalises mboxes (e.g. order them by time), but seems risky for something handrolled.