Do you mean "Choose from among permanents other than ~ one of those with the greatest mana value among them"? Because that's slightly different in that currently this won't destroy e. g. a Serra Angel for having greatest mana value since the first choice will just become impossible (greatest mana value is six not five, so it wouldn't be a legal choice).

Also do you really want to allow this to destroy lands? They'll be the options for the lowest mana value for a lot of the time.

I feel like this should exile or force a sacrifice so it doesn't get stopped by an indestructible option.

Destroying lands was intentional. That's why I cost it so high