If you want “en” to be used, then the site default locale would make sense to be “en”.
If you disable the setting, there won’t be a fallback and the user will just see the original language (unless it is translated to a supported language, per the description).
A Finnish visitor who has set their browser locale to “fi” (highest priority) will see “fi” (if they don’t, it’s a bug, let me know). I feel like you can go ahead and set your default locale to “en”.