As a daily user can I share my "bugs that drive me mad"...
Having read the thread so far, I've minimised the duplicate topics you've already covered including:-
RCS, VoLTE, notch/corners, slow GPS fix, backup/restore, small buttons.
Below is in personal priority of annoyance. Hopefully this ramble is helpful.
Clock app:-
In my opinion the clock/alarm app is terrible and annoys me daily. Stopwatch and Timer are much better to be separate better fleshed out apps. Missing timezone information for world clocks but hours ahead and behind useful feature to keep.
I would suggest having the top menu options as just World Clocks and Alarms windows, loosing the swipe up menu completely (see below). Having a separate settings menu for each. Alarms menu could have larger font for time more legible and note be one line to maximise space for alarms list, as too much wasted space.
General UI Swipe ups:-
I'm not sure about any of the swipe up menus across all of the apps. Yes they're mostly consistent to be "add new thing" but really can be achieved much quicker and intuitively with a + on the top menu in every case. Swipe up is not an easy gesture to do either often requiring two hands as you need to swipe at least 1/3 up the screen to open the window.
Only swipe up that isn't "add new thing" would be the phone app which I feel should just display recent calls list anyway with options to open either Contacts or Dialling pad.
Settings:-
Lack of network connection options frustrating, proxies, static IP's, DNS, automatic connection, priority, etc.
Needing to connect to things with static IPs is one reason I need to keep a laptop around.
Previous networks should only show if you successfully connected to that AP.
General UI PullDown:-
Calender from pull down shows incorrect dates for future months. Multiple times I have given incorrect dates due to this and now have to open calendar app not use the pulldown one.
Keyboard Layout not consistent with settings. With English UK set in settings, a bluetooth keyboard types English US and is shown in the keyboard pull down as US.
Phone:-
Better highlighting of missed/incoming/outgoing calls from Recent calls would be useful read at a glance. Red/Green/Blue text maybe?
VOIP or IM accounts for Calls and Messaging. No XMPP/Jabber?
Music app:- Resume playing on headset reconnection needs to be an option.
Not using local album art seems to be a bug, I have folder.jpg files for all my music and yet thumbnails rarely work.
Now playing gestures for play/pause (to go with swipe skip gesture) would help for driving.
Nice to Have:-
Adaptive Charging or Maximum % threshold should be available in battery settings.
Lack of Printing app as standard?