Update to my last post: the Timberland Boot Kit helped a bit, with a few cycles of brushing and cleaner and then conditioner. But what ultimately helped most was dried bread. :
Timberland Premium 6-inch rubber toe waterproof boots in black | ASOS
Cleaning & Protecting Timberland Boots | Official Guide | Timberland UK
How to Clean Timberland Boots | Cleanipedia
How to Clean Timberland Boots With Soap and Water in 3 Steps
How to Clean My Timberland Boots | Timberland - YouTube
Timberland Boots Cleaning Package– Sneaker Garage
Timberland Premium Waterproof 6 Inch Boot - Rust Nubuck
How to Clean Timberlands, the complete guide - Alex Righetto
How to Clean My Timberland Boots | Timberland - YouTube
Timberland Premium Waterproof Boot Review - Is the Hype Real? - stridewise.com