I want to be respectful but if they say they don't care what pronouns I use for them, that feels like it puts the decision on me to choose what to call them and I guess I would probably default to "they" because choosing a gender for them feels weird... am I wrong?
"They" is perfectly acceptable, in fact I've taken to writing work emails as such too. Because you can't always work it out through their name alone.
The real problem is not learning or correcting themselves when told.
"This is Alice, she...".
"Oh he is a nice pers...".
"She, mum".
Be open to correcting yourself when told, people are used to correcting people. But continually not correcting yourself makes you look like a bigot even if you don't mean it.