[HTC 10 ROMs, Kernels, Recoveries, & Other Developm] GROOVYJOSHCLARK:
This is a very extensive and well written list of bugs/issues, most are already known. I will list known under each one:

Changing the brightness slider from 0% to 50% does nothing (screen brightness does not change at all). Screen brightness only changes when going from 50% to 100%.

KNOWN

Adaptive brightness sometimes changes too abruptly instead of smoothly changing from one level to another.

If the toggle for adaptive brightness near the slider is enabled, the brightness slider width will be cropped on the right side. For instance, if you set the brightness slider to around 85% and the toggle is enabled and you pull down the quick settings, it looks like the brightness slider is set to 100% but then you start to drag the handle and the adaptive brightness toggle will be hidden to reveal the rest of the slider.

KNOWN

Removing the "Android Keyboard (AOSP)" from the available keyboards and using Gboard only (installed from the Play store) seems to cause an issue with the Gboard's "Haptic feedback on keypress". I have this option set to off but if "Android Keyboard (AOSP)" and I reboot the phone, I get haptic feedback when I press the keyboard keys on Gboard. Checked the setting and it remained off, enabling and disabling it again, disables haptic feedback but on the next reboot, it will have haptic feedback again. None of this happens if I keep "Android Keyboard (AOSP)" in the list of available virtual keyboards.

Not as well known, I also use GBoard but do not have this issue so I cannot comment on a solution. I have it set pretty much the way you do but I dont have the vibration, I disabled it in GBoard settings and it remained set for me.

Trying to change "Settings » Display » (Advanced) » Style » Accent color" or trying to apply "Automagic" crashes (logfile here).

KNOWN

Launching "Settings » Display » (Advanced) » Expanded desktop" also crashes (logfile here).

Some settings on "Settings » Display » (Advanced) » Ambient display" like "Double-tap to check phone" and "Lift to check phone" don't seem to do anything ("always on" is disabled), along with "Tap to wake" in the previous screen (outside of "Ambient display").

KNOWN

Do we need to enable anything on Ex Kernel Manager for these?

User choice

Not sure if this is how it's supposed to work but the "Dark" "Device theme" (under "Settings » Display » (Advanced)) changes the quick settings to a dark although notifications are still white. Feels a bit weird and inconsistent but it's probably how it's supposed to be, yes or no?

Kind of known, not all users try it, but it doesnt work yet

"Settings » System » Status bar » Network traffic monitor" is not working.

KNOWN

In-Call volume does not change when pressing the volume up/down keys. Am I missing some configuration, doing something wrong or is this a know problem?

KNOWN (but doesnt affect me, mine works but others have issues, some do, some dont)

Not sure if related but In-Call speaker volume is too low and cannot be changed too.

People on the other end of a call hear echo of themselves talking.

KNOWN but only affects certain users/devices, I have NO echo, others have NO echo but we have not yet found the similarities to the ones with the echoes and ones that doesn't, what is your provider, this is helping us rule down the differences.

I also have a few questions that I couldn't find answered anywhere:

Is it possible to change the "LTE+" status bar icon to the smaller "4G" one without using something like Xposed?

You would need substratum or swift but this is user choice

Do you guys recommend to use the built-in battery saver or the one from Ex Kernel Manager?

I dont but others do, I recommend not tweaking to much in EX for now, wait until more updates come down from upstream LOS

Just to be sure but, by using Lineage OS, there's really no need for a Magisk module like Pix3lify, right?

Everything that module contains is already baked into Lineage OS, right?

User choice, but not sure what you mean by "baked in" if you are asking if the upstream contains everything from LOS, I believe everything upstream is merged downstream into this OS, you can read the changelogs

Since SELinux is not yet enabled, do you guys disable the system alert you get about this when rebooting the devices?

Yes, I disable alerts and play protect as I use apps outside of the play store