Best bang for buck? That’s kinda like asking what the best screwdriver is… It depends. In this context (but not in all), you should focus on Zone 3 repeats.
Twenty-seconds is too hard and too short this close to a skimo race. For training, they would be best far in advance of something like the PoF. With only four weeks before the event, you want something close to race pace.
Check out our How & When for High-Intensity. Focus on the Z3. Good luck!