After a bullet leaves the barrel the speed of its spin gradually slows down right? And the slower the spin gets the less accurate it is. If I made a bullet that held the speed of its spin throughout its entire flight it would be more accurate at longer distances right?

Yes

I don't think the frictional forces against spin are significant over the flight time of a normal bullet, which can be spinning at up to 5 kHz. If you spin it too much faster - the bullet can break up from centrifugal forces.

Capuchin is absolutely correct, and in any case the complexity/cost involved in getting the spin to remain constant (supposing it to be even possible) would outweigh any theoretical benefit to practical applications.