For anyone who needs it, here is an article on rocket mass heaters: https://insteading.com/blog/rocket-mass-heaters/
The photos show heaters that are fancier than most of the ones I’ve seen, but the basic idea is the same. Use small-diameter fuel such as twigs, and run the smoke through in a way that captures most of the heat and radiates it to you slowly. That allows you to heat the same space with less fuel, and allows you to gather firewood with pruning shears instead of a chainsaw.
I’ve seen some that were basically a large wooden box built around a horizontal chimney, and then the box was filled with pea gravel.
Also, if firewood is hard to come by, there are alternatives. Dry corn cobs make decent firewood. The disks of a sunflower burn like charcoal briquettes. I’m sure there are others, I’m still experimenting.
Photos of those heaters at your link are great!