How is your tuner going to be semi-automatic, if I may ask?
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Suggest you consider a stepper motor from a dumped
printer/scanner/floppy drive or HD ,if necessary including a shaft
rotational reducer when steps are too big.
Google will provide you with suitable (simple) circuits to drive the
motor ,usually involving one or more opamps.
From semi-automatic the circuit can evolve towards a fully automatic
system with a SWR sensing circuit.

Frank KN6WH/GM0CSZ