Graphner I have installed GrapheneOS multiple times via an A-to-C cable. Use a good cable. Also, on some machines different USB-A ports work better/worse (as noted in the instructions).
If an installation attempt goes wrong, just unplug the phone, hold the power button until it turns off (maybe more than a minute), and then try flashing again.
The web installer is pretty darn good. And by this time the installation instructions are quite solid. People most easily get into trouble by skipping steps or reading some detail and not knowing how to do what it says and figuring it's probably ok to skip that part. Sometimes yes, sometimes no. Also, it is best to avoid doing extra steps that are not in the instructions but that frequently appear in videos. For example, there is no part of the installation process that requires USB debugging.