Probably not possible (for security reasons) to stop a systemd start-job while booting. From this post:
Q: how can you have control whilst booting? IIRC my keyboard ctrl+c break attempts do not work.
A: Well, you cannot, for security reasons... We cannot allow altering
the boot process just from the screen without authentication...
If you run "systemctl enable debug-shell" you can turn on a debug
shell on tty9 that runs continously from earliest boot on. But that of
course can only be done on the previous boot.
As mentioned in above thread it might be possible to use a debug shell (if it is already enabled), but this is only recommended for debugging and not for permanent use. From the systemd documentation:
Early Debug Shell
You can enable shell access to be available very early in the startup
process to fall back on and diagnose systemd related boot up issues
with various systemctl commands. Enable it using:
systemctl enable debug-shell.service
or by specifying
on the kernel command line.
Tip: If you find yourself in a situation where you cannot use
systemctl to communicate with a running systemd (e.g. when setting
this up from a different booted system), you can avoid communication
with the manager by specifying --root=:
systemctl --root=/ enable debug-shell.service
Once enabled, the next time you boot you will be able to switch to
tty9 using CTRL+ALT+F9 and have a root shell there available from an
early point in the booting process. You can use the shell for checking
the status of services, reading logs, looking for stuck jobs with
systemctl list-jobs, etc.
Warning: Use this shell only for debugging! Do not forget to disable
systemd-debug-shell.service after you've finished debugging your boot
problems. Leaving the root shell always available would be a security
It is also possible to alias kbrequest.target to debug-shell.service
to start the debug shell on demand. This has the same security
implications, but avoids running the shell always.