Decryption isn’t a problem if you use the systemd hooks when creating your initrams. They try to decrypt every given luks volume with the first key provided and only ask for additional keys if that fails.
I have 3 disks in a btrfs raid setup, 4 partitions (1 for the raid setup on each, plus a swap partition on the biggest disk), all encrypted with the same password.
No script needed, just add rd.luks.name=<UUID1>=cryptroot1 rd.luks.name=<UUID2>=cryptroot2 rd.luks.name=<UUID3>=cryptroot3 rd.luks.name=<UUID4>=cryptswap
to your kernel parameters and unlock all 4 with one password at boot.
ARM is shit at hardware discovery in general. So no, chromebooks don’t need a special distro. They however need a kernel adapted to the specific hardware, often down to the model (that’s also the reason Android updates take so long on phones and there is very time limited support… there’s always someone needed to adapt new updates to the specific hardware for each device, so they don’t bother for anything but their latest products).