For the Oxide Grey on Hyrule, there is a portal to some island trees at Paka’s Palace, Alcyon, off TNT Alcyon. T6 more wear of course, but with those trees, if you dig underground just a bit, there is a layer of solid wood all around the trunks. The trunks themselves are hollow but easy to farm wood right under the surface.
Edit: Also this farm though not sure if still up - T6 Farms: Horizontal Sap / Bark / Lustrous - Safe & Efficient - 4 farms - 4 COLORS!