peachy custardy cobblery crumble

desdemona brought up the idea of making this kind of a dish and it turned into kind of a call for experimentation among myself + jordanpattern. i have been feeling really stifled in the kitchen lately, like i've lost the ability to improvise, and this was just the thing i needed to get my cooking juices flowing again. it wasn't a complete success, but the part that wasn't successful didn't impact the rest of it and the rest of it was SLAMMIN'.
here goes:

peaches:
5 medium peaches, peeled and sliced
1/3 c. demerara sugar
1 t cinnamon

custard (optional):
4 T bird's custard powder
3 T demerara sugar
3 c. rice milk, plus a few T extra

crumble topping:
3/4 c. AP flour
1/2 c. margarine, cold
1/2 c. light brown sugar

preheat oven to 350.
combine the cinnamon and 1/3 c. sugar. layer the peaches in an 8x12 baking dish and sprinkle the cinnamon sugar mixture over them. bake for 15 minutes.

while peaches are baking, prepare the custard. combine the custard powder and sugar in a bowl and mix well with a splash of milk.
heat the rice milk to nearly boiling and pour into the custard/sugar paste. mix well and return to the pot, heating to the boil.
here's where my problem came in: my custard never set. i'm not sure if it's because of the rice milk, if i might've accidently left out 1 T of powder, if i didn't heat it long enough. but it was thick over the heat and thinned out once it cooled a little. like, too thin. i'll keep experimenting with it, and it tasted fine once all done, but i think the custard part is totally optional because the peaches would be fine with just the crumble topping.
pour the custard over the peaches, just to cover them. you might not need to use the entire 2 pints of custard.
bake for 15 minutes.

while the peaches + custard are baking, prepare the crumble topping by mixing the flour and sugar and cutting in the cold margarine. mash with a fork, your fingers, whatever, until everything is well combined and crumbly.

increase oven temp to 400.
sprinkle the topping over the peach/custard mixture and bake for an additional 25 minutes.

then i let it cool down and topped it with a little soyatoo. oh, man.
SO GOOD.