Hey buglover, that's a seriously cool project! Building an actual calibrated SLM in Pd is pretty deep stuff.
For the dB(A) filter, I'd definitely recommend checking if any of the external libraries you're using (like iemlib) have a pre-cooked filter for the A-weighting curve. Trying to implement that IIR from scratch can be brutal, as you found with fexpr~!
And yes, absolutely go for the RT-kernel on the Pi. For any real-time audio and especially for continuous logging like an SLM, having that low-latency stability makes a huge difference and will save you endless headaches down the road.
You might find this free tool helpful. https://noisemeter.co/