Is it permissible to pay a financial expiation for many missed days of Ramadan fasting, given a medical excuse (severe dry throat and choking) and the fear of death before making them up?

It is not permissible to substitute feeding for making up missed fasts unless two conditions are met: that the illness is not expected to be cured, and that fasting causes an unbearable hardship. If these conditions are not met, then it is not permissible to switch to feeding. If you are temporarily unable to make up the fasts until another Ramadan begins, then you must make them up after it, and there is no expiation due for the delay.