Look.
The one thing I know with 100% certainty that transwomen don't think is necessary to be a woman is a female body.
And that means that whatever this new and improved meaning of the word "woman" is that now also includes male people, it's not especially relevant any more to the needs and lived experiences of the people who do have female bodies.
We might overlap in the venn diagram, sure, but we overlap with various male bodied people in all sorts of ways (hobbies, jobs, clothes, politics, music, nationalities, race... pretty much anything other than the things directly determined by our bodies really). So "woman" just becomes another of the many things an individual female person may or may not be, and may or may not have in common with male people.
So, I don't actually need to know what "being a woman" specifically is to trans women, because just by knowing what it is not I already know that the thing that I consider significant and important in the original definition of woman is excluded.
I already know that whatever it is it has to be mental rather than physical, so I already know that it falls under the old sexist ideas about some ways of thinking being inherently innate to and correct for "women" and other ways being inherently innate to and correct for "men".
And that means I don't need to know exactly what it is to nevetheless know that trying to replace the original female-specific version of "woman" with this new male-inclusive version makes it less relevant to the realities of female people and also to reduces the diversity of "women"'s minds into a sexiat subset of our true diversity.
Yeah, not signing up to be reduced to that thank you.