Signal's Mark read action points to an internal broadcast (MarkReadReceiver, action org.thoughtcrime.securesms.notifications.CLEAR), but you can't fire it with am broadcast from ADB: the intent carries a threads extra that is an ArrayList<ConversationId> (a custom Parcelable), which am has no way to construct — and the receiver is app-internal anyway. (MarkReadReceiver.java)

Android/LineageOS also has no cmd notification subcommand to invoke a notification action, so the practical ADB route is to open the shade and simulate the tap with uiautomator:

# 1. Expand the notification shade
adb shell cmd statusbar expand-notifications
sleep 1

# 2. Dump the view hierarchy
adb shell uiautomator dump /sdcard/ui.xml >/dev/null 2>&1
adb pull /sdcard/ui.xml . >/dev/null

# 3. Find the "Mark read" button bounds (text= or content-desc=)
B=$(grep -oE '<node[^>]*(text|content-desc)="Mark read"[^>]*' ui.xml \
    | grep -oE '[0-9]+,[0-9]+][0-9]+,[0-9]+' | head -n1 | tr -d ']' | tr ',' ' ')
x1=$(echo $B | cut -d' ' -f1); y1=$(echo $B | cut -d' ' -f2)
x2=$(echo $B | cut -d' ' -f3); y2=$(echo $B | cut -d' ' -f4)

# 4. Tap its center
adb shell input tap $(( (x1+x2)/2 )) $(( (y1+y2)/2 ))

# 5. Close the shade
adb shell cmd statusbar collapse

Notes:

• If your notifications are grouped (your dump shows groupKey=g:messages), the button may not be in the shade until you first tap the collapsed "Messages" group to expand it — dump the UI again after expanding.
• If the first dump comes back empty, the shade was still animating; just re-run steps 2–3.
• The Reply action (RemoteInput) is likewise not reproducible via am broadcast — it also needs Signal-specific parcelable extras — so tapping is the only clean ADB approach for it too.
• If uiautomator dump fails entirely, the fallback is blind keyevent navigation: open the shade, input keyevent KEYCODE_DPAD_* to focus the notification and its action button, then KEYCODE_ENTER/DPAD_CENTER — much more brittle than the dump-and-tap method.