2. If by low frequency you mean that it takes too long for the circuit to reset itself for the next button press, decrease R1 and R2 to 47K each and see how that works for you.
1. Check the 5551 data sheet for continuous collector current, and compare that to the calculated collector current with the change to 8 ohms. For long-term reliability, you want the actual current to be less than 50% of the datasheet value.
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