A Kobo ereader is as open as I need it with koreader installed. The Kobo Libra 2 black and white is the best ereader I have ever had. (not the color version,,unfortunately a step back in reading quality for normal reading)
I got the colour after being on an old paperwhite for many years so thankfully it's a big step up for me.
They don't compete, but do complement. Both are very open and hackable, so you can use the same process to get books on both.
You really don't need to build a Kobo clone. By being open, you can fit your needs. If your needs are already covered by Kobo, that's fine.