We had a bitter background taste when 1st started smoking a few yrs back.
The came across a Friend of a freind who is retired after yrs of professional smoking and now does craft smoking on a semi pro basis..
Listened for couple hrs..
!st thing he said was dump the manuka.. when he now does a manuka smoke he throws a little in with the other woods just to be able to say "manukau" as it is a big pull for marketing propaganda.
And that is also the reason a lot of ladies dont like smoked fish...
So We dropped manukau long time ago, experimented with apple, maple, feijoa, grape , peach, pohutukawa, miro, loquat and hickory and blends of them
Hickory we also dropped due to a slight background bitterness.
A lot of the woods came from our gardens...
We also found that if we roughly stripped most of the bark before putting thru the big mulcher, that cleaned up flavours significantly.
Our go to for now is a feijoa / apple/ maple mix
And thats for fish, muscles, scallops home made bacon, chicken, sausages, pork belly, pork and lamb chops etc.
We have never cleaned the smoker out other than trays grills.. thu be very careful if condensation from any chimney is avoided.. we have a S bend in the chimney with a hole to drain any. If this gets on the food I would not call it bitter.. its just a real crap taste.
Any old accumulated meat/ fats laying around will also give an off background taste... espec if finishing off things like chicken by increasing heat at the end of the smoke.
Our general smoking temp is 48 to 52 with 5 to 7 hr smokes.. latter being things the size of large king fish slabs.
PS don't use iodised salt.. it will give a very faint taint, and will only notice if in the same smoke use uniodised on some items and noniodised on others.