OP is right to consider landfill. Haven't you heard of Greta Thunberg?
Autistic people can be big overthinkers - and that's actually a strength, not a weakness. She actually THOUGHT so hard about what the right present was, she couldn't think of the right thing. So sensibly, in my opinion, she bought nothing.
OP, I have not bought presents for people in the past and for kids at birthday parties.
Sometimes I've felt it was a waste of money or I was just too busy to find the right thing. Part of my thinking is informed by my opinion that we do live in a very throw away culture. You're right to be worried about landfill.
Why does a kid need yet another £1 set of crappy felt-tips. Who is that helping? Can we not all just put our endless trivial needs to one side and think about the environment for one second. You saw how hot it was this year. Where is the line going to be drawn between endless frivilous crap that makes your child feel special for all of 2 seconds and then ends up in the ground somewhere for thousands of years.
And when I've got a difficult response like you have, I buy a gift and bring it in a couple of days late. Then they have to eat humble pie. I'm not the big bad mean ogre they thought I was.
And overall I'd say you did extremely well. Everything you did was spot on.
Instead of dressing you down this appalling woman made you feel bad. It really is her at fault not you. And if you don't want to share your diagnosis with her, why the hell should you??
She isn't worth your time. Big pat on the back OP from me and 10/10. I wish I'd been there as I would have enjoyed chatting to you. You sound right up my street.