I looked at Merriam-Webster, Oxford, and even Merriam-Webster's Unabridged Dictionary. "Ill-qualified" is not a word offered in any of these dictionaries. However, the use of a hyphen connects the two words and makes a new adjective with a combined meaning, like "just-shined Chevy" or "My I-literally-just-cleaned-him dog went in the ...
One simple (and the shortest) way of rephrasing the sentence so that it unambiguously provides the meaning you want is the following:
I have booked a September flight.
This turns September into an attributive noun, leaving no room for misinterpretation.
The use(s) of of with quantifiers
The usual function of "a couple" can be described by the label of "quantifier".
Other words that can act as quantifiers include numerals, more, and all.
Many quantifiers cannot directly precede a definite noun phrase: we can't say "twenty the children" or "more the children". Since a ...
In this case "fully" is not needed as "electrically" already describes powered. But if you really need "fully" in the sentence, i'd rather say, "This car is fully powered by electric." instead. The only time to say "fully electrically" is when someone believes that the car is powered both by gas and electric.