Hello! I dont have the experience with it or the proper data, but my guess is that microdosing MDMA can't be healthy in the long run.
The way LSD or most psychedelics work is by being parcial agonist of some serotonin receptors. Serotonin itself is not disturbed by them, but they link to the same places serotonin is supposed to link to.
On the contrary, MDMA acts as a serotonin releaser: It takes the serotonin that's already in your "storage facility" and floods your system with it. It doesnt create more serotonin, it uses what's already in there.
My point is: Using traditional psychedelics doesn't "waste" your stored neurotransmitters, while MDMA does. That's why its only recommended to use it 2-4 times a year at most, so you can build back your storage again. I guess using it on a regular schedule, even in low doses, can lead to serotonin depletion, serotonin receptors downregulation, and depression.
P.D. Somebody with more technical knowledge, please contribute with more details
Edit: Please don't mix ayahuasca and MDMA. That's a dangerous combo (not because of DMT, but because of IMAO containing plants)