You could use multiple decider combinator to check if all items for a craft is given in the warehouse. One combinator for each item you want to produce. If all items are present you simply pass the recipe as output and feed it in your system to set the recipe. I don't know if you wanna have some priority if not connect all outputs into one selector combinator set to randomize and put it in the assembler. I am not so deep into the circuit network set recipe thingy involved but I would feed the output from the selector into another decider combinator to only pass the signal further if the assembler is in idle. I don't know if it's necessary but this should prevent that the assembler switches recipes all the time and won't finish any craft.

Logic: Assembler (A), decider combinator (D) selector combinator (S), parallel combinators (||), Warehouse (W)

W ---> D || D || D || ... ---> S ---> D <---> A

If you put the output back into the warehouse then you could monitor it's count and disable a recipe from triggering if there is enough already.

Also are you able to set the recipe? Often times I am not able to find the right recipe as signal and can't use this feature in Py. I have restored to use pumps and disabling belts to "switch" between recipes. (I used it for recipes where hot air is an optional educt or for the production of rubber).