Gather everyone with a precommitment like Rains, take the half that donate least per month, add their donations per month, make a collective loan that can be paid off with a monthly payment equal to that sum, have everyone pay off a ratio of the monthly interest equal to the ratio of their monthly donations vs the sum of all participants' monthly donations, and donate the rest. This will work without problems if at least half the participants continue to pay. If this isnt enough, decrease the collective loan until the probability of failure is under 10^-10.

Alternatively, I advise everyone with a precommitment like Rains who also have a savings account on general principles to empty it into donating now and filling it back up over the next years/decades with the monthly donation.