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.
