https://github.com/koreader/koreader/issues/1731
You can add a "ksmAutoselectafter=0", to the options list. With KSM7 you would add it to the General section of; ".adds/kbmenu/confoptions/ksm_ini_options.txt" , I would suggest using Notepad++ to make the addition, it formats the page in a much more readable way. You should still "return" to KSM when you exit KOReader. (With KOReader set to open in the current book, you would have a pretty quick return from Poweroff.)
Luck;
Ken