OOOOOOO right there with you. It really takes less effort to not use the phrase ever too, and I can never think of a time that I heard it where "but they /are/ X" isn't just as if not more respectful (provided that mentioning someone being trans is a respectful/necessary detail to include within context).
"They were born as a boy but they're a woman now." Vs "They're biologically male but identify as female." Like you can practically FEEEEEL the calipers in someone's hand when they say the second thing.
Fuck even just "well my sister /used/ to be my brother" in casual conversations where that detail is relevant (and she's out btw) communicates the information just fine (and also it's a little funny every now and then when it takes someone a second to put what I'm saying together imo lol).