On reflection: It's not a problem to write a first draft with poorly-written characters. I worry about it. That worry slows down my writing. But I don't think it actually matters.
I've often worried if the characters I'm writing "work." I worry they're inconsistent and nonsensical or else soulless wooden things. I worry they don't make sense as people. I worry that writing characters poorly means I am an inattentive, thoughtless, bad person. I don't think this is actually true—but that's anxiety.
I think this anxiety has slowed down my writing. I write sentence by sentence in part because that's what I have time for. But I also have long blocks of time when I could write, and instead end up doing other things—like spending ~8 hours making a list of vowels and consonants I don't know and want to—so it's not entirely practical. I sometimes write longer when I feel inspired, but that's not reliable. I feel like I could set aside time to write for say one hour once a week, but I'm disinclined to, even while I've set aside regular time for other activities. I think this is why.
The worry does nothing to improve my writing. In fact, it hinders it: I produce less, get less feedback, and pay less attention to the feedback I get lest it confirm my characters are poorly written. I catch myself reading multiple writing tips in series, rather than reading one tip and putting it into practice, because reading feels like positive action on the worry and doesn't risk writing a character poorly.
On reflection, this makes no sense. Worrying doesn't help me write better characters. I have to practice. I have to try out devices to make the characters work better. I have to try planning and thinking through the characters more. I have to write stories, get them in front of people, and get feedback.
In the past, I've taken the worry to represent a real problem and tried to solve it. Maybe it is useful to take it less seriously. Maybe it's useful to write characters poorly—especially in a first draft. After all, it ought to be possible to fix it in a second draft.