My sense (coming from a place of complete ignorance) is that the difficulty here is not going to be software/control side, but rather the hardware. It'll have to be capable of:

1. Accurately dose the required ingredients based on a setting selected.

2. Do so in a repeatable manner. 5mg is microscopic and likely well beyond the margin of error for a small dose pump. So you'll need to get creative in how you manage your product dosages. Perhaps a "reel" of pre-measured 1mg doses? You say it's not for Nicotine, so I assume it's for harder drugs. What will the risk of a double digit % overshoot be?

3. Be small enough to fit into a handheld vape module.