I always found this argument funny because how would you use pronouns for someone whose gender you do not know? They. It's they. E.g. you are given the sentence: Jordan went to the store to buy apples. And you want to ask a followup question regarding how many, you reply: How many apples did they buy?

And that's not how English was taught to me or 99℅ of the population (including English as a second or third language) 20+ years ago. Singular they was only used for situations where the gender (read as superficially visible sex) was factually unknown. You see a forgotten umbrella and never saw who forgot it: "Somebody forgot their umbrella." As soon as you only got a glimpse on the person forgetting it you would make a guess about he/she.

They has been used for gender ambiguity in everyone's lives since grammar school.

If you're younger than ~30 and from Great Britain, maybe. GB were the first to formalize and teach it like that less than 2 decades ago (if I recall correctly).

People just have an inherent bias towards trans folks and it's incredibly depressing and sad.

That's bullshit projection.

I, a non-native speaker, complain about increased ambiguity of the language because of singular they as a personal pronoun and make a proposal about new pronouns for the purpose.

You: Ah, must be transphobe. Let's ignore everything he said (which doesn't relate to transphobia at all).

It's so frustrating not to be able to have a discussion about stuff making a language harder than it needs to be without people invoking transphobia, like, instantly.

But hey, I called it: can't have a discussion about it and I've given up on it.

edit: tiny add-on. I was still taught gender-neutral he and only heard about they later while being discouraged to use it in writing.

It hasn’t been used for “gender” ambiguity, but sex ambiguity, because the separation of sex and gender is only a recent thing and it’s not even unanimously accepted.