Can a Standard User format an encrypted drive?
You're right- while booted from the other partition you don't need admin access to erase a disk.

You can set a firmware password to prevent someone from changing the startup disk if you're worried about that. But keep in mind they could just physically remove the disk from the computer. The data will still be encrypted but the firmware password won't follow the disk (pretty sure).

Setting a strong user password and, of course, storing your recovery key in a secure location are important for securing your data.