Since the introduction of those bonus quests, I spent a full supreme alchemy tool and got no proc, so I was wondering about other experiences and trying to figure out the proc rate of those quests.

I've gotten the alchemy bonus quest. Artisian level 9 at the time.

I was working on making 2-30 batches of oils-regeneration, fortitude,storms, tranquility, corruption etc,etc, for oil of the void and other crafting products.

Number of oil batches depended upon amount of ingredients.

I finally got my first bonus quest after using almost 2 supreme alchemy tools. I'm master 26 and I was making wings elixir.

Perhaps the chance of the bonus quest appearing could be increased a little?

It sounds like it may be at a very low chance currently.