I tried koreader on a kobo. I wouldn't call it trivially easy when I installed it last year, and I promptly uninstalled it and removed all the hacks so I could get back to a sane installation of the Kobo OS again.
I think maybe for a kindle it might be worth it, but the reality is for Kobo, it's probably more hassle than it's worth.
I found my time better spent setting up a calibre-web in a docker container and then having my kobo sync to that. And that was awesome.
What's the issue you had? When you reboot or quit you're kicked back to regular Kobo and you open up Koreader through a menu option.
The Kobo OS is good enough for me to recommend it to family, there's nothing wrong with the iPod experience of uploading books.
But Koreader+Tailscale+Kavita OPDS is the best reading experience I've ever had.
I don't remember frankly, maybe something with the script, maybe something with KFMon. Lack of integration with the Kobo OS itself? Maybe it had to be rooted to install KFMon, but I was reading Koreader could just run inside nickel menu maybe? But yet the script installed KFMon?
Again, I don't remember. And whatever it was it's not worth me trying to reinstall it just to remember what it was, and then to uninstall it again.
The installation instructions are a bit windy, so many people miss that there's a simple script that automates the install for you.
The one thing you have to watch out for : You need to return those scripts whenever Kobo has an update. You won't lose your data or anything but a standard Kobo update dialed disables Koreader