I stand corrected, its been quite a few years since I needed to use the ADB backup so I guess back then it was more complete.
Found a good detailed explanation here.
Shame that it seems to be getting phased out for cloud backup solutions. Makes sense that google would want to control more of your data and make you pay for it.
As installing a custom ROM typically involves using ADB anyways, I would suggest that you back up your device normally (copy files over to a folder on your computer), and then use the built-in backup function in ADB to make a secondary complete backup.
Also, depending on your threat model, you might not want to move any files from your old installation to your new one. Its possible that the old files, applications, and linked accounts could compromise your new installation privacy / security. I also generally enjoy starting with a clean slate after a new OS install.
GrapheneOS also has this cool feature called Scramble PIN Layout to try and protect against guessing the pin from fingerprints on the screen.