Pulse train profile generator for stepper drive step/dir input
I would use a microcontroller with better fine delay control such as something from the TI MSP430 series. They can generate the pulses directly.

With an RPi, if you only need rough control, try a LM566. It is a voltage controlled oscillator with a 10X range. That is, the frequency can be varied 10x without changing the RC values. That would seem to meet your needs.