The ChromeOS equivalent of LineageOS is usually just installing a normal linux desktop distro, sometimes one that's been been tweaked to have drivers specific to chromebook hardware like GalliumOS (https://galliumos.org/)

I don't think "a normal Linux desktop" matches the same target audience as those who would wish to run ChromeOS

And yet, there are a lot of early adopters of Chromebooks who modded them to share the kernel with a full linux environment, tied to a key toggle, while still using the lighter browser-as-OS for its strength, web browsing. Myself included.

Keep in mind, Google's reputation among hackers has dropped a lot in recent years. Trying out an operating system where the core premise was "you spend 95% of your time on your netbook just in a browser, why not optimize for that and strip away the rest" was an intriguing proposal. This was back when doing things in web apps instead of desktop apps was new and forward-thinking, not totally overdone the way it is now.

For x86 chromebooks there's https://mrchromebox.tech/ to replace the firmware with a normal UEFI-booting coreboot, and then you just install whatever you want on it because now it's basically a normal PC.

For ARM chromebooks I'm only aware of https://archlinuxarm.org/ and that's still pretty patchy; I hope that something comes together (or that it already exists and I just haven't found it yet).

There's FydeOS (dunno much about it other than it came up in a search for "CloudReady ARM", CloudReady being the name of ChromeOS Flex prior to acquisition) or build-it-yourself ChromiumOS.