I find it kind of neat how the sand-plankton/sandtrout/sandworms are a fully self-sufficient ecosystem of one species. But yeah, I have to assume the sand plankton are doing some sort of photosynthesis equivalent, otherwise it makes no sense. I don’t think actual photosynthesis would work for them, I doubt sunlight is going to penetrate very far into the sand and there doesn’t seem to be a noticeable concentration of them in the near-surface layer. Maybe the sand plankton are capturing energy from ambient heat via some sort of biological thermoelectric generator?
Maybe the sandworms/sandtrout/sand plankton really do spread around the universe by hurling their spores into space or something (they aren’t originally native to Arrakis)? One species being able to establish an entire self-contained ecosystem would make sense for an organism like that, as it would allow them to spread to a wide variety of planets. A planet they land on doesn’t need to have pre-existing life and they wouldn’t need to be too picky about atmosphere etc., they just need a planet somewhere in a relatively broad range of temperature, air pressure, composition, etc. and then they can establish their own new ecosystem and Arrakisform it from there. Maybe there are thousands or millions of planets like Arrakis scattered through the galaxy? Even the high numbers for the size of Dune Imperium put it at only a very small fraction of the galaxy; 13,000 inhabited worlds is around one star in every 15 million in the galaxy - so totally possible there are thousands or millions of Arrakis-like worlds humans haven’t found yet.
Regarding the teeth, I think that’s kind of a fanon thing (in the sense that the Lynch movie and so on are fanon); the books mention knives carved from sandworm teeth but I think maybe those “teeth” can be interpreted as something more like more baleen.
You’re right that the sandworms seems really implausibly aggressive. I guess they’re kind of like sand kaiju hippos, but that level of extreme aggression seems kind of weird when they’re not predators and there’s basically nothing in Arrakis’s natural ecosystem that could be a threat to them except each other other. Maybe they’re paranoid-aggressive because they’re cannibals? I mean, of course they’re cannibals, the sandtrout and sand plankton they live on are their own immature forms, but I mean maybe they opportunistically supplement their filter-feeding by eating smaller sandworms? Then it might make sense for them to see anything that seems like maybe a big animal as a threat, cause they’re the only megafauna on Arrakis so in their ancestral environment anything big and loud was probably another sandworm. Still seems kind of thin though. Maybe…
One kind of meta interpretation I can think of is the sand plankton/sandtrout/sandworm ecosystem of Arrakis is a kind of superorganism that has a kind of intelligence and recognizes human civilization as a threat, and the sandworms are acting kind of like an immune system. Think about what might happen if they weren’t that aggressive; without a need for the caryall system spice mining could be much more efficient and Arrakis might be much more built up and might be basically getting strip-mined or at least mined much more intensively, damaging the ecosystem. The extreme aggression of the sandworms is protecting the natural environment of Arrakis, keeping the planet in a mostly wild and relatively undisturbed state. It also protects the Fremen culture in a similar way; the open desert being so hazardous is what allows them to be so isolated.
This superorganism might be running a lot of its intelligence on sandworm neural tissue (maybe that’s why they’re so big!), so maybe sandworm waste is basically LSD as imagined by hippies who were really into the idea of using drugs to “expand your mind” because it’s loaded with discarded sandworm neurotransmitters and the like. Taking spice gives you superpowers because you’re absorbing broken discarded bits of this planetary intelligence and some of those bits are still somewhat functional and start doing their thing inside you.