They could just make a fixed day, like the last Wednesday of the month, or something like that.
This would be the best option to implement and it will not bring repetitive complaint/question every single month.
They can do it for the weekly bonus so it should not be a big problem for them to implement it for the monthly bonus.
Are you serius 50-70$ from rainstream? Hmmm.. i never get it until 50$ from rainstream. Maybe you get 50-70$ from your wager every weeks brother.
I guess you have never tried to claim the bonus drop during the weekend live stream (which is also being shared by @rainstream in telegram).
Usually there are 5-7 bonus drop in every stream worth starting from $3-$7 for the normal drop and $5-$20+ for the last bonus drop.
Your wager during the last 7 days does effect because there is always minimum wager requirement in order to be eligible for each drop but as ryzaadit said above, in average you need to wager $7k-$9k in order to be eligible for all codes.