Is there a way to go through all routes, unlock all illustrations, and accomplish all achievements in one account? Or do I need 3 accounts to do so? Is there a way to restore my purchased contents in other accounts?
No way of restoring. You'll be able to fully restart at the end of season 2. Tier 3 patreon members can switch routes at any point. All achievements can be acomplished without patreon or multiple accounts. Ilustrations do require tier 3 or waiting at the end of season 2! Hope this helps!