With outdoor timber and durability, you either have a choice between treated pine and expensive hardwoods. Pine is treated with CCA, which contains arsenic. This is how it kills bugs, fungus etc. The problem is that at the end of its lifespan it has to be disposed of as a hazardous material. It can’t return to the earth, can’t be burnt it cannot even go to a landfill. In America, CCA is now banned. Hardwood options like Balau, Garapa, Cedar, Spruce are all imported (in South Africa) and are often harvested from endangered tropical rainforests and boreal forests.
In South Africa, there is a product called Rhinowood that is made from local, sustainably harvested pine and is treated with a biodegradable wax. It does not need to be oiled or varnish and has a class 1 rating for durability (which means it will last 30 – 50 years).
In 4 years it has faded to a silver-grey patina, which I love and requires no sanding, painting, oiling. Which suits me just fine.