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Hi Bojan,
If it all gets too complex I’ll suggest there is a simpler solution along the lines of a controller I made in the 1980s using just two CMOS ICs and a handful of components that fitted in a tiny box. You can solder it on a prototyping board (hard way IMHO) or use wirewrap, this is quick and easy.
In essence ... wire up a CMOS 7555 timer with RC components and a pot as Alex suggested to give a nominal tracking rate variable by +/- 50% and feed that into the STEP input of a stepping motor controller chip. Wire to motor and power supply with a diode for reverse voltage protection and a fuse in it in case something is shorted. For the variable resistor use a decent Bourne wire-wound 10-turn type, not the rubbish from Jaycar as these fail rapidly.
You can add some hardwired logic for fast/slow push button from a handset if so desired.
The RC circuit will be good enough for visual since there’s no attempt to correct for PE or add autoguider inputs (which are feasible BTW).
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