But isn't that basically what they did? They took the known phenomenon of mycelial networks and extrapolated a transport system out of them
No, it's completely different, because it's an inept analogy. Sure, yes, mycelial networks are structurally analogous to networks of interconnection found elsewhere in nature, but that doesn't make them
literally the same thing. It's the sort of thing you get when someone who doesn't understand science listens to a science lecture and draws the wrong conclusions from it, and I'm just left there shaking my head and going "That's not how any of this works."
Okay, yes, they did make up the idea that this particular kind of fungus exists partly in subspace, which somewhat justifies it, but it's a patch on an intrinsically silly idea. And the fact that they're now apparently treating the network as this sentient pseudo-afterlife thing just makes it more woo-woo and mystical.
(using the completely made-up phenomenon of subspace which is also used to explain warp travel and transportation- ie they are equally ridiculous).
Wrong. There actually is a physical concept called a subspace -- it's a subset of a certain number of dimensions within a higher-dimensional space. Yes, the concept as originally used in Trek (cribbed from earlier prose SF) was just a handwave, but TOS only used "subspace" in reference to radio. The first time the term was associated with warp drive was in Dr. Jesco von Puttkamer's technical notes for
Star Trek: The Motion Picture, in which he defined "a subspace" as the bubble of spacetime within a warp field, using General Relativity-based physics that were essentially identical to the "warp drive" equations Dr. Miguel Alcubierre would derive some 16 years later. Puttkamer didn't work out the actual math, but the principle was equivalent.
However, the use of the term "subspace" in early SF, and in TNG and after when it was established as another spacetime domain, is basically a variant term for hyperspace, a concept which is entirely grounded in real physics theory of higher-dimensional spacetime continuums. Don't assume there's no science behind something just because you don't know the science behind it.
That is not 'making up random gibberish', that is more explanation (an explanation more connected to a real, observable phenomenon) than is given to Q, Trelane, the Guardian of Forever, Nagilum, Apollo, Organians, etc.
And I hate most of those concepts. I've never liked it when Trek skews more toward fantasy than SF. I've explained why already, and I have a right to my own tastes and opinions. You don't get to tell me I'm wrong for not liking the same things you do.