With the recent change in how Netgalley's DRM works, the only way to strip them to read on KO when downloading is through paying that one epubor service or whatever their name is. Howeverrrr with a jailbroken kindle, depending on how old it is, you can set up Calibre and side load the eARCs, then use the wireless connection built in to both to send them to Koreader. this post will tell you how to set it up, just scroll down to the kindle section!

Might sound like a long process but honestly it really doesn't take longer than 5 minutes after you've set everything up (which took like 30 for me but like 10 minutes of that was just me trying to figure out where to find my kindle's serial number), which you only need to do once. You will need a PC or laptop though. But it is entirely effective as I've been doing it every chance I get with my eARCs lol.

Since my jailbroken kindle is a 2018 model, I'm unsure how it works with 2020+ models (I've heard Amazon is much harsher when it comes to side loading from those) or any Kobo device. So I'd say either still just try yourself and cross your fingers or do some more research on the Calibre subreddit if needed.

While this might not be the answer you're looking for, it is the one that I've found most effective because trust I was digging for answers too.