I did have KOReader on the Elipsa for PDFs till a FW version that did PDF crop sensibly.
I do sometimes use it on the Likebook Mars as its native epub is dire. Oddly its PDF reader is better than KOReader!
But I'd not use it for epub on Sage. I'd fix broken epubs and wife would have benefit too. KOReader for epub, IMO, is to override broken epubs. You loose the advantage of My Books (filters and categories) and Search that is so much better than Kindle if you use KOReader. Having collections, titles, authors, series, unread / reading / finished and Recent etc on My Books via Calibre Metadata is so easy. The KOReader is painful for searches in comparison and by default slowly browses real folders rather than sort & filter by Library Metadata. Also I use highlight & touch keyboard annotate (big highlight, small note) and export that via Kobo Utilities to a tabbed text editor.
We read a huge amount on the Sage and battery life is not an issue. Front light, WiFi and BT are off.
Also we have 3m long (10ft in American) USB-C cables that cost €2 ($2 in American
) and a charge from 10% to 90% is less than 45 minutes. She'd not go back to Libra or Paperwhite 3 and she never uses note books.
Two other family members have Sage (4 total). No complaints. All seem identical quality.