How to calculate delay/phase caused by filters and other modules.

"How can I calculate the number of samples delay that are happening in my effect so that I can compensate for it in the "dry" side?"

You can't use a delay, because that is frequency independent. What you need is an all-Pass filter that complements the filter.

There is an example of this in the core filter library. If you look at the crossover filters, there are some macros with a += in the name, these mix the bands with the dry signal, so the dry signal needs to be phase adjusted to match.

e.g. here is the X-over LR2 += macro (2-pole Linkwitz-Riley crossover mixer)

The all-pass applies the same phase offset to the dry signal without changing the amplitudes, so the phases align.

You need the correct all-pass for the filter. Might be able to do this in Primary with guesswork, trial and error and luck. Or in core by doing the math (ouch)… or by using the TF (transfer function) section of the filter toolkit to plot the phase curves of various filters and match them to all-pass phase curves.

(not sure how you deal with something like a 2pole band pass where the phase offset goes from +90 to -90 though…?)

Here is a basic setup to plot the phase using TF (no actual numbers here, just the curve, but it's still useful ;)

Using this it's easy to see that a factory 2pole SVF low pass (EDIT:with resonance at zero!!) has the same phase plot as a factory 1 pole all-pass. Just by swapping the connections (to the funny 'j' macro with the |z| and arg outputs) and seeing the same plot.

Seems there are fancy things like minimum phase filters, and fir zero phase things etc. but for standard iir there is no way to have no phase distortion afaik.

It does make some intuitive sense that if you have a multi-mode filter then mixing the low-pass output with the high-pass output should give you all-pass with the correct phase, but I'm sure there are some subtleties in there that I'm failing to consider :))