Well, the TRA position is that gender is a social construct and simultaneously, people are born transgender.. If you're born transgender, gender is surely biological, not a social construct? If by their own admission, gender does not exist outside of social and cultural influence, of which a new born baby has never experienced, then how could a baby have been born transgender? Going even further, presumably a baby still in the womb at 9 months is transgender? Even though gender is a social construct. That's kind of the issue with gender ideology, it's full of logical fallacies and contradictions. If gender is a social construct, you open yourself up to the idea that transgenderism has a social contagion aspect to it, which they would never want to acknowledge.
It doesn't really matter whether you think the difference in men or women's behaviour is biological or socially influenced, or both. The point is "terfs" don't want womanhood to be defined by stereotypes of femininity. They don't want a certain behaviour to be considered inherently female, a certain role in society be inherently female, because it reinforces stereotypical gender roles and makes them to defining aspects of womanhood. The whole point of identity in the first place was uniqueness and exclusiveness, of which biological sex achieves. There is nothing inherently unique to males or females about social aspects of being "feminine" or "masculine", it's simply more common in one or the other. If gender is a social construct imposed on us by the patriarchy, why would we want to reinforce it by making it a form of identity, and even more, our primary form of identity?
The fact you're as hard left as they come and you're still sitting on the fence around this surely tells you something about your true feelings on the issue?
And as for "Terfs" being right-wing adjacent and therefore not credible. The irony is if you disregard a radical feminists opinion because it happens to be shared by the "right wing", you then have to acknowledge that the left shares 95% of their political views with radfems. Does the left want to acknowledge that they agree on almost every political point with the hateful bigots that are radfems? What does that say about their credibility that they're aligning themselves with radfems on almost every issue (with the exception of gender ideology)?