Breathing life into some disposed of ChromeBooks

So I'm helping a local tech non-profit refurbish some old Chromebooks for distribution to halfway houses and immigrants that need computer access for legal stuff. The current need is basically a rock solid platform for getting to websites, reading email, and editing mostly shared Google docs.

Issue is that the hardware is no longer supported by Google.

We've gone ahead and got Coreboot flashed on all 40 devices and have settled on using Fedora-Onyx (Atomic distro with a Budgie UI).

We need to install some flatpaks on each machine and set up a base configuration. Easy enough with rpm-ostree and some manual configuration, but I was wondering if anyone here has had more experience with managing the atomic distros.

Basically I want to have it so the volunteers just need to plug in a USB installer stick and get a fully setup instance. Is there an easy way to take a tree and transfer it to another machine that isn't using something like clonezilla? I'm assuming we could just maintain an image and rebase to thatafter installing, but I'm not fully aware of the easiest way to accomplish that.