Chromebooks are locked down yes, but they do give you the keys. It involves unplugging the internal battery to be able to modify the hardware write protection, entering dev mode to disable the write protection, and then flashing a Coreboot port onto the firmware. Even then, a lot of basic things may or may not work once you’re booted into Linux. From experience I don’t recommend.
You should try the Shimmer userchrome tweaks along with the Sideberry extension. With both of them it’s even better than Zen IMO.