Interesting conundrum. The problem with cardboard and paper (which can use other forms of cellulose, such as old cotton) is not the sustainability of the raw material (easy enough to plant replacement trees) but the environmental footprint of production, which for first use materials is something like 4 times that for "plastic", at least in terms of shopping bags. (http://www.allaboutbags.ca/papervplastic.html) The extra volume and weight of the materials used is also a factor, and sees paper bags costing more in shipping, and also having a larger environmental impact in terms of waste. They also don't always last as long.
Not sure how that translates to materials used for modelling directly, but feeling (self-?)assured about reducing one's environmental impact by using card and wood bought new is questionable. If the materials were produced for other purposes - anyone else remember John Ahern's references to cardboard cartons? - then fair enough, but as the plastics we use are largely a byproduct of what would otherwise be waste materials from the oil refining industry or possibly leftover off-cuts from more "worthy" manufacturing, then I am not sure that using Evergreen polystyrene (my preferred manufacturer) is really doing a great deal of damage, compared to say driving to an exhibition, which I know you no longer do so all kudos to you! But what about the polystyrene you are using in your lightweight baseboard construction? Was that waste, or bought specifically? (We know each other well enough, I hope, for you to take that as a prompt for reflection and discussion rather than as a jibe!)
To follow Ikigai, you would need to produce your own wood, card and paper using waste heat and your own hard work, which is not what I think you are aiming for? The ultimate execution of this philosophy would be to not build a model railway at all, merely imagine it in one's head; we all do that already!
I am all for environmental awareness, but I think we need to concentrate ourselves on where we can make major changes to our lifestyles, rather than inconsequential ones that appear to be a grand gesture.