You can just read the experiences of actual trans people. A common thread is a sense of dysphoria with their own sex and associated gender expectations. It's not a new invention, but a well-understood one. Gender non-conformism has existed for as long as human beings have and it's well-documented throughout history. I don't think this phenomenon is especially contentious - most people have encountered others who don't fit with our gender expectations. Mostly, it's not that hard to understand, some dickheads might say shitty things and abuse them, but they agree on the basic facts.
What's contentious is the unique Western viewpoint that non-conformity and dysphoria are the result of a mismatch between the ego or, if you like, 'spirit' of a person and their physiological sex - that there is a woman 'trapped inside' or 'born into' a man's body, and vice versa. As a theory, it's shot through with holes and contradictions, as is the assertion that reshaping the body to match the inner 'spirit' is. In science, the best theory is the one that accounts for the greatest number of facts and no single theory (outside of mathematics) is so watertight that it's beyond replacement by a better theory (Newton's gravity was replaced with Einstein's; Einstein's will one day be replaced with someone else's). This isn't how many modern gender activists view their theories - they view them as an article of faith, contradictions and all, and accuse those who disagree of heresy.
It's perfectly possible to sympathize with a person who feels at odds with the societal expectations placed upon them by their gender (most of us feel that way from time to time). You can credit the sense of dislocation and dissociation they feel without crediting a conspicuously medicalized (in the Western world, who'd have thunk?) explanation and treatment that almost exclusively prescribes expensive surgical and pharmaceutical interventions, some of which will require lifelong medicalization.
Gender non-conforming people are not 'trans' by default, 'trans' is merely one of many different theories that seek to explain the cause of the non-conformity and as explanations go, it's not the best. Moreover, it's the only one that comes with a market opportunity built in at ground level - that it's the preferred theory of one of the most aggressively capitalist countries in history is probably just a coincidence.
Also, as final question: is it possible for a non-trans person to understand what it is like to be trans? If yes, then why can we not have valid opinions on that? If not, then how does any person (who's not already trans) discover they're trans (if they are)? If the feeling is indescribable, how would you know what you felt was what another felt?