tl;dr In the Quick Switcher input box,
Ctrl
+Home
andCtrl
+End
will bring the cursor to the start and end of the search string, respectively.
(Obsidian version used: Obsidian for Windows v.1.5.12)
Perhaps this is common knowledge and I apologize in advance if that is the case.
This answers the following unfortunately unanswered but closed topic: Home and End keys do nothing in Quick Switcher