Welcome to Myce.
For M Disc, this article is a pretty interesting read on how well it copes when deliberately left out in the weather. I tried a similar experiment in the past comparing various recordable CDs and DVDs and every disc was ruined after a month, however, I have not tried it with an M Disc.
High humidity damage is progressive. Even short periods of high humidity will cause damage over time, much like repetitively placing a film through an X-Ray scanner will progressively damage the film bit by bit. Maintaining a low humidity level will not reverse damage, but it will prevent any further degradation.
To keep the humidity under control, I suggest placing the discs in plastic storage containers with a large fresh (300g+) rechargeable silica gel pack and a digital hygrometer placed up against the side of each container. Check the reading every couple of months and recharge the silica gel once the reading hygrometer reading approaches 50%.