As you will have gathered, there are no BIFL recycled or plant based leather boots on the market. So if you want something sustainable than buying a second hand pair of a really high quality chelsea boot and cherishing them is the best option. It is unfortunate but at the moment veg leather is absolute garbage, buying disposable shoes made of that stuff is not something a friend of the earth should consider.
Depending on your needs you could consider a canvas + rubber boot.. Palladium has some nice options, not BIFL but they are decent quality for the price and most of their boots are free from animal products.