Hi Alex,
"Transgenderism" doesn't exist. The suffix "-ism" is given to groups of people who have coalesced around a set of core ideological ideas.
There is no such "core ideology" in the case of trans people. The things that group us together are the twin facts:
1. We are trans.
2. We exist.
"Transgenderism" is a slur invented by transphobes in an attempt to negate one or both of those facts by reframing it as if it is a mere opinion worthy of debate.