Hmm both rooms should have different spawns of snakes, not share the same spawn timer.
The first mission is to unlock 2nd room (tele pad) with 100 venom.
Possible add a 2nd quest in the 2nd room to kill x amount of snakes for 50k exp to unlock third room.
Third room will have final quest to defeat manda (150k exp).
Therefore, snake lair will have a cool quest line instead of locking Manda to lvl <30. @Ueda what do you think?