Have you tried recording a macro and then assigning it to a shortcut?
e.g. Your replacement could be:
Find what: (\A|[.!?]\s+)(\w+)
Replace with: $1\u$2
Tick 'In selection'
And then navigate to Macro
→ Modify Shortcut/Delete Macro...
in the top menu and assign a shortcut.
This is the resulting macro that I extracted from C:\Users\%USERNAME%\AppData\Roaming\Notepad++\shortcuts.xml
.
It uses the shortcut Ctrl + Shift + C
<Macro name="Capitalise" Ctrl="yes" Alt="no" Shift="yes" Key="67">
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam="(\A|[.!?]\s+)(\w+)" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam="$1\u$2" />
<Action type="3" message="1702" wParam="0" lParam="896" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
</Macro>