I feel like last minute preparation actually made my presentations better. YMMV but if you actually have a decent grasp of the subject matter speaking is not a problem, your slides will be condensed and have only the necessary info. Some of my peers had slides upon slides of text, verbatim from their oral part. You don’t need 20 slides in a 10 minute presentation.
Less is more and just wing the oral bit, just put bare minimum salient info on the slides which will serve as key talking points if you forget something.