Yes, the way I like to think of it is that plants and wood and stuff like that from established tanks at least "can't hurt" in terms of possibly giving a very slightly higher chance of releasing a few of the beneficial bacteria into your tank. But unfortunately they do surprisingly little compared to an actual transfer of mature media from a mature filter. Putting in plants also carries the complication that they will need light and its "light plus ammonia" that promotes algae, so a new problem potentially presents itself.
You are correct that organic material can be used as the (indirect) ammonia source for a fishless cycle. Its indirect in that the material, be it fishfood or prawns/shrimp or other organic matter breaking down, must be attacked by heterotrophic bacteria (a different set of species of bacteria that are omnipresent in fresh water and are not our beneficial ones we need in the filter) and broken down into ammonia. This is a very, very unpredictable process in terms of timing and amounts, leading to a pronounced lack of control over the fishless cycling process and this is why we work quite hard in the forum to encourage fishless cyclers to keep aggressively seeking the bottled ammonia. The bottled ammonia is just many times easier to use to do the whole process. But, technically, you are correct that fishfood or prawn cycling is a fall-back of sorts.
One of the problems when you lack control over the ammonia is that if the ammonia concentration gets up around 8ppm or higher for an extended period, a different species of ammonia oxidizing bacteria can be encouraged and can out-compete the Nitrosomonas spp. that we are trying to grow. This leads to a big time setback because this wrong species has to die off and give up its media spots so that the correct species can take over. (You'll note that in reality there's really a lot going on with quite a few different bacterial species in a freshwater tank than we generally discuss due to the complexity confusing people.)
Sitting over here in the USA and trying to imagine you in Thailand I have thoughts of you boldly approaching some labs at a university or going to a drugstore pharmacy or something odd like this as one of your things to try. I'm sure that's harder than it sounds...
~~waterdrop~~
