Enable ttyS0 and ttyS1 on all systems. Also enable the first serial
console for GRUB.

Still, the virtual consoles (tty1, tty2, ...) are kept as default
consoles. Technically /dev/console remains connected to /dev/tty0, which
in turn is always the currently active virtual console.

You can log in on all of these, but systemd's boot messages only go to
/dev/console. (See https://github.com/systemd/systemd/issues/9899)