Power is dependent on the force you put through the pedals and how fast you can turn then. For a good sprint you need both leg speed and strength. The low cadence intervals will build strength, and the low power form sprint or spin ups will build leg speed. I only do proper full on sprints outside, but the low cadence efforts and spin ups are easily done indoors.
There’s a few things that I’ve been recommended to do…
On a long ride incorporate 5 min blocks in a big gear (53/39 if you can turn it) at no more than 50 - 60 rpm. There’s lots of TR workouts with these kind of intervals. Strength sprints, where I’ll go into 53/17 and slow down to about 8mph then sprint full gas for 15 seconds. That’s a very intense effort so lots of recovery inbetween. I’ll only do this outside. I’ll also do short VO2 max intervals at high cadence. Again, lots of workout on TR that do this. I’d read that Mark Cavendish always finishes his road rides with a long sprint so I started doig that too!!
I incorporated this kind of stuff during all my training, indoors and out last winter. Improved my sprint no end.