When using a custom banner, I get the error dashboard-insert-banner: Symbol’s value as variable is void: file-exists-p. This is the setting logic:

  (setq dashboard-startup-banner (if shan--settings-exist?
                                     shan--preferred-logo
                                   'logo))

I've confirmed dashboard-startup-banner points at the right file absolute path using M-: and printing its value. I've also confirmed this isn't an issue with the startup hook by killing the buffer and using dashboard-insert-startupify-lists, which results in the same error.

Using

  (setq dashboard-startup-banner 'logo)

works fine for the logo, which I'm using for now.