First on wood, some is toxic on its own. Coniferous wood (pine, spruce etc) should never be used for this reason, the "sap" is toxic in an aquarium. Oak and beech works, and probably most hardwoods, but some (certain flowering shrubs/trees) may not be safe.
Second, I agree with the members who suggest not boiling the wood but simply using it. The slower release of tannins will be more effective long-term. But having said that, in most cases this does not affect the water chemistry much, regardless of the colour resulting. It would take a lot of wood to significantly lower GH/KH/pH, and this also depends upon the initial GH/KH.
Leaves work well; I have been stuffing my 10g with dry oak leaves collected every autumn in my back yard. I do this not to lower GH/pH but as a prime food source for the
Farlowella vitatta fry that are growing up in this tank. The water is clearly tea-coloured. Photo attached shows this tank to give an idea of how many leaves; second photo for interest shows two of the fry, a larger fry mid-photo and a newly-hatched upper right. When I drain one pail (3 gallons) of water during the water change, in a cream-coloured pail, the water is so brown I cannot see the bottom of the pail; it doesn't seem this dark in the tank, but obviously it is.
Peat pellets is another option. Again, depending how low you want the pH, and the initial KH, you need to work out how many pellets and how often they need replacing. And as with leaves, these do give out, depending upon the afore-mentioned, so none of these methods is long-term unless the substances are regularly replaced. Diluting the water is the only long-term fix.
Someone mentioned using carbon to remove the dark colour in the water. This is true, but at the same time you are removing much of what you are adding. Carbon adsorbs (not absorbs, note) all sorts of substances, including DOC (dissolved organic carbon) which is crucial to plant growth, and you earlier mentioned plants in this tank. And DOC creates CO2 which helps to lower pH, so you don't want to be removing it. Never use carbon in a planted tank.
Byron.