Filtering Surge Signals
Sounds like a job for dedicatedly identifying these surges and then actually subtracting them.

I'd start with a low-pass filter to find the slightly time-varying mean of the signal.

Use that to define lower and upper thresholds above or below you count something as surge.

Identify the samples lying outside the thresholds. Find a signal model for surges, e.g. a (truncated) sin x/x function or a quadratic function. Fit that model to the surge samples and then subtract from the signal.