Why would you answer the phone?
You're a busy person.
Just don't engage. You've got enough on your plate.

Tell her you are done discussing it, yes, and that you need some space as you come to a decision.

Tell her you appreciate that she is concerned and trying to help, but it’s draining for you to keep thrashing it out. Ask if it’s ok for you to run things by hercwhrn you need feedback but otherwise the way she can help right now is to be a source of breathing space from the whole thing.

I've been in your situation and I would answer the phone "Hi Mom, it's good to hear from you. I don't have an update on whether we are starting Larlo's therapy so let's talk about something else. Anything new on Netflix?"

you don't want to discuss it

You'll have to rely on others, not your Mom. Going forward make sure you have other support. Do not burden her. You describe her behavior as not rational. If she knows, she can not help but have opinions and will want to tell you what she thinks.

Totally tell her. "Mom I know you mean well but this is not helping. Please stop asking and pointing out the problems. I'll let you know when I've made a decision."

Just avoid her calls. You don't need to pick up the phone just because she is calling you. Especially multiple times a day. Maybe call her back once every other day or so.

I am trying to make a difficult decision regarding my child with disabilities. My mom is calling every day (or, like today, multiple times a day) to ask me my current thinking on this decision. I find talking with her about this exhausting because she points out the problems with whichever decision I make (every choice has problems about which I'm well aware, that's why this is such a difficult problem). She thinks she is helping and she means to help, but it's just exhausting.

Would you tell her flat out that you don't want to discuss it (which is likely to make her upset and/or defensive) or continue to avoid her calls (which is what I did today)?