I think the approach often depends on the interaction. I have taken an approach similar to OP's original position when talking to my young-adult nieces. They are all about (what they understand to be) intersectionality. They believe that TIMs are among the most oppressed of women and those of us lucky enough to be born in the right bodies have cis privilege.
It would absolutely have been counter productive to scoff (aloud) at the idea or insist that TIMs are men with male privilege. Instead I asked them why, if they could see that TIMs suffer oppression both for being perceived as women socially, and for being 'born in the wrong body', so many transactivists (like those who object to the term fgm or criticise womens marchers for using reference to the female reproductive system in their slogans and imagery) refuse to think intersectionally when it comes to women who experience oppression both because of being perceived as women socially, and because they have female biology and physiology?
If we are all just different types of women, why isn't the fact that some of us grow up treated as sex objects from when we start to develop as girls, experience shame and embarrassment over our periods, have to fight for control of our own reproductive systems, live at risk of male violence and sexual assault, are treated as an afterthought by the medical establishment, have traumatic experiences exacerbated by disrespectful services and public perception when it comes to infertility, childbirth, breastfeeding, etc. etc.... Why isn't this viewed through the lens of intersectionality, and those of us who suffer oppression on those two fronts, actively supported and 'centred' by our fellow 'women'? Why instead are those of us with female biology expected to shut up about the oppression we face on that front and only concern ourselves with the struggles we share with TIMs (btw what struggles are they anyway?) and with being good allies and checking our 'cis privilege'?
Is any other group of women who are oppressed on more than one basis, expected to shut up about the experiences that are specific to their group? Or is it that they disagree against all the objective, clear evidence and their own lived experience, that having female biology is a basis for oppression?
I feel certain that our conversation made them think in a way that insisting that transwomen are not women would not have. I didn't get much by way of response, but I hope that when they next come across someone pissed off that a woman marched while holding a sign mentioning ovaries, or whatever, they will have a think about which side of that particular argument they are on.