On the SVJ try to really stretch UP with your arms extended overhead and then load and jump. You have a very small amplitude of movement the way you're doing it right now.
You can time your sprints yourself using a video camera at the finish in front of you... you can then multiply the number of frames in your sprint with the associated time per frame and get your result.
Well... the most I ever used was 40 kg dumbbells but could only do 5 reps because my hand would just unfold and drop the dumbbell... I even used 45 kg ones for 3 reps.
It's never been a leg strength issue for me with dumbbells. But I kinda like to use them because I also challenge my grip strength so...