Hi OP, hope this helps you understand:
Gender critical and radical feminists see 'sex' as the concrete biological difference between male and female bodies (men have testes, women have uteruses), and 'gender' as the two sets of constructed social roles that are imposed on each sex respectively, by means of socialisation (boys like blue, girls like pink).
We're 'gender critical' because we believe that it isn't the first sex distinction between men and women that's harmful: it's the social roles (gender) imposed on them afterwards. These roles disproportionately hurt women, and also disproportionately hurt people who are very likely to identify as trans - butch lesbians and feminine gay men. We want to get rid of these roles, thus helping trans people, while still acknowledging the biological differences between men and women.
Instead of putting trans people on a path to lifelong, untested medication and lining the pockets of pharmaceutical companies, we would prefer them to do something more radical: to own their natural biology while still embracing everything that makes them, them. Instead of seeing your hatred of female gender roles as something that makes you male, you'd see it as something that makes you brave, a trailblazer and an activist.
In a truly gender critical world, nobody born female would even consider opposing the term 'woman' to describe themselves because it will be completely neutral, with no implications: it will say nothing about their values, interests, personality, likes, dislikes, career or fashion taste, and simply describe the capabilities of their natural body, only having relevance in matters of medicine, sexual intercourse, sports and safeguarding. The same would be true with the term 'man'.
If you have any questions I'm happy to answer.