I'm easily overwhelmed by noise and hustle-bustle at times, mostly when my health conditions start flaring up in the middle of the parks, so I've found a few quiet places. My top favorites:
Bottom level at Hungry Bear (especially good when adding funnel cake fries to the quiet time!), though sometimes the top level is quiet at non-meal times and allows a great people watching opportunity while still being chill.
Tom Sawyer Island (there's lots of quiet-ish nooks and crannies where you can overlook a fairly quiet Rivers of America).
Tomorrowland bathrooms next to Autopia. Yes, I know it sounds weird, but it's typically fairly quiet-ish back there, and if I'm really needing a break and am in that area, it's an easy spot to get to.
Big Thunder Mountain Trail. Even with the entrances to Galaxy's Edge, the little "off shoot" areas give a fair space for a breather.
Under the Silly Symphonies Swings facing the water. Caveat: *NOT* the place to be when they are doing a Magic Key swag giveaway, lol. Then it can be chaos.
The trail behind Grizzly. I've even attended work Zoom meetings/trainings back there.
If it's the heat that's cranky-ing me, then I'll just hop from indoor show to indoor show to indoor show. I can sit through multiple rounds of Turtle Talk since it changes based on the crowd interactions, and it's right across the street from Philharmagic.
I know plenty of people like the Main Street Cinema, but that's always a hit or miss for me; I've been in there interrupting canoodling teenagers, toddlers throwing tantrums, and kids running around in circles.