I would use pyton.
Just find the proper script to remove keys from the track. You have the time interval from my quick math above.
it should be simple enough to loop through the animation by time intervals and remove the in between keys.
I’m currently doing something similar but with the bones instead.
Not sure how you got to that many keys to be honest. It must have been a fat finger setting mistake on the capture or during the keying process.
Hope you find the right pyton commands and can sort what would otherwise take hours in 20 min or less. Best of luck.