Dihydroxymethamphetamine is very easily oxidized to the quinone, which will polymerize with the amine group (and other stuff).
Formaldehyde cannot be used to form methylenedioxy bridges, a dihalomethane must be used. This will also rect with the amine in basic solution.
Your synthesis of DMMA will work though.
https://www.rhodium.ws (https://www.rhodium.ws)