Installing Gentoo on Vim3

I recently bought a Khadas Vim3 Pro. I really would like to install Gentoo on the system. But how to get started? I can access the debug console and can flash the Gentoo ARm64 iso to USB or SD. But how to make it boot up? I’m always ending up with Android running from the internal eMMC. I’ve also tried fiddling around with krescue and entering “upgrade mode”. I probably also need proper dtb’s, but I don’t know where to download these.

Help appreciated,
thanks in advance.