Calling it gender ideology.
This is because calling it trans ideology both centres trans people and makes them feel like they're being attacked or like we object to their existence, when the issue is the very different way they understand gender as a concept. It makes it clearer that it's the belief system and not actual trans people's lives that is what's up for debate.
I also think that referring to gender ideology as something I don't subscribe to as opposed to telling people I'm gender critical has made me personally feel more comfortable about being more open about my beliefs. This is because declaring myself gender critical sounds like I've joined a new movement, which suggests that I've made a change and might therefore have decided wrong, whereas I want to make it clear that my views haven't changed at all and it's simply that I don't believe in this new ideology doing the rounds.
I also think it's really important to refer to it as an ideology because (1) it's under the category of belief that we receive protection under the Equality Act and (2) the sooner people accept that gender ideology is a belief and not a fact, the sooner they will realise that they're being incredibly unreasonable in trying to convert and bully others into believing what they believe.
On the same theme, I tend to use the term gender ideologues nowadays instead of TRAs, mainly because more of the people I come across tend to be non-trans proponents of gender ideology and the belief system has spread to a wider range of people.
I also feel gender ideologues is a more accurate description of the people pushing this stuff nowadays as I wouldn't class most of the people who have accepted gender ideology as activists at all; quite the contrary, I feel that many people have simply sleepwalked right into putting pronouns in their emails and changing their language without even really thinking about it too deeply at all.
I realise that gender ideologues get upset when you call it an ideology, but that's exactly what it is as it's based on faith and not science. It isn't falsifiable and by its very nature can never be proved as fact by science. There is nothing wrong with believing in whatever gender myths you fancy, but pushing your beliefs on others and bullying non-believers into submission is completely unacceptable, and referring to it as an ideology hammers this point home.
I also feel that referring to it as an ideology helps me to remember that even outside of the EA protections, I'm perfectly within my rights to resist religious conversion. No-one can force me to repent and accept gender ideology as fact, and framing it in these terms helps me to avoid some of the gaslighting as it helps me to recognise more clearly the situations where someone is bullying me for my lack of belief as examples of extreme religious zealotry.