My mother is a chronic complainer and I can't seem to find a peace between us

My mother (60+) and me (29 F) are fairly close in terms of familial relationship. The issue is that my mother will always default to nitpicking and complaining about most anything that comes up, frequently interrupting our conversation we're having. She'll very often talk about something casual then it'll spiral into her nitpicking that topic then complaining, which usually offloads pent up frustration she has from her day to day life onto me, which makes me start pointing it out and asking if we can just talk about something else or maybe nothing at all. I usually point out her nonstop nitpicking when it gets to be too much, which is frequent unfortunately.

I usually get around 10-15 of her asking "why this" and "why that" until I point out her complaining and usually me pointing out how I really don't want to listen to it right now. It doesn't seem to matter in the moment how I approach be it gentle, patiently, calmly. It doesn't really work. She gets very defensive and tells me to "stop talking about it already" when I point out how it's exhausting me to listen to. I often default to try to figure out the root of the frustration which gets me to go to therapist mode.

It's just an odd and somewhat hard to describe issue that I can't seem to figure out. She's constantly complaining about anything. Our dog, our house, the people walking down the street, people driving and how stupid they are, how the cats won't stop meowing at her, how the neighbors exist loudly and such, and on and on and on. If anyone has any real advice not "your mom's a narc, move out" it's not that black and white so please do not give such black and white answers.