Securely deleting the keys is safer than simply practically inaccessible. That suggests their is some impractical way to access the data, care to suggest what that might be?
Overwriting flash with 0s. To be fair I think the iPhone should grant you that option (MacOs does). But to suggesting that leaving an encrypted block of data on the device is on the same level as leaving unencrypted blocks of data lying around is foolish.
Overwriting flash data isn’t going to give you access to the data. It might in theory be more secure, but I would favor encryption at rest rather than simply overwriting plain text.
It is possible in theory to recover some information from overwriting flash data with zeros, it’s also possible in theory for well designed modern encryption to be broken but neither of those are guaranteed at this point.
Essentially every encryption scheme in history has been broken. It's a bit silly to think that "well designed modern" makes it safe. The others were all well designed and modern at one time, too. The only scheme that's truly unbreakable is a one-time pad and that's... well... not really practically useable.
Personally, that's not a risk that I would care about (especially in the context of an Echo Dot or even an iPhone), but deleting the keys will not be better than inaccessible forever.
