As a professional computer geek I like to tightly control variables and not change more than one thing at a time if I can help it. At the same time, I don't want to have you keep turning good fish into trash fish.

So... if it were me, I'd do the following steps one at a time and evaluate each of the results.
1. As already determined, use less wood. Alder sounds like a common choice for fish. Hickory has a stronger flavor and I wouldn't start out recommending it. However, I'm assuming you've successfully smoked with this combo in the past and know that you like it, so carry on.
2. Join the great experiment and see if preventing your wood from catching fire will resolve the bitter flavor. One of my first three smokes resulted in the wood getting burned up and turned to ash. That also spiked the temperature in the smoker. At the time I wasn't really paying attention to know what impact it had on the product... I don't even remember what I smoked. Regardless, I was alarmed, read some posts here, and took action that has resulted in nothing but charcoal post-smoke since. I simply line the bottom of the smoke box with a double layer of thick aluminum foil and poke smaller holes where the existing holes are. I like a very strong smoke flavor and often use more wood than recommended, but I don't recall ever getting a bitter flavor as a result. I'm also a black-coffee drinker and an omnivore, so maybe I just don't have a refined palette.
3. Maybe lower the temp a little? I'm thinking 175ish. I suggest this with some hesitation because I don't have a solid explanation for why beyond reviewing some old posts suggesting it for fish.
Everything else you're doing sounds perfectly reasonable and normal. Keep us posted... I'm really anxious to hear about your future success!