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Trying again with the PRB as a base for my turbo k24a4, and so far I have it running incredibly well. Tuning is harder having only 2 VTC tables, but no more lean condition in boost and it cruises like a stock vehicle.

Cold starting is difficult, seems like there isn’t enough fuel or ignition is overly retarded, it turns over but stumbles & struggles to actually fire up. Typically takes 2 tries before it will start, but after it finally catches it runs & idles flawlessly.

The only setting I could find was the cranking fuel, which I have been increasing slowly but it hasn’t seemed to help at all.
Is there another setting I should be looking for to fix this?
(Engine fired up immediately & effortlessly with every PZD tune, so it is something inside the PRB)

I don't think there's a need to create new threads every time you have a new question related to your same setup, as this appears to be ongoing. There is an ECT fuel cranking enrichment right where you'd expect to find fuel related parameters, in the fuel corrections section.

I figured breaking out the various issues would make the info easier to find for others, but I can stop.

I have gotten the cold start a little better by tweaking most of the ECT fueling tables to match the PZD values for the k24a4. It’s still slow to start with a longer than normal crank time, but it’s acceptable for now.

I am struggling a little bit with the PRB’s different VTC table options.
K24a4 is limited to 20* VTC & with the PZD I had 0*, 10*, & 20* tables, so I could tune for the extremes (0* & 20*) for controlled interpolation between the tables, with 10* as a midpoint table.
With PRB I only have 0* & 15* tables, so how am I to approach tuning for fuel & ignition when VTC angles are between 15* & 20*?
Isn’t the ECU then interpolating from the 15* & 30* tables to derive a 20* VTC value?

It is interpolating.

So how do I tune the interpolated areas when I have limited VTC tables to work with?

(Practical application of software question, not for tuning theory)

If you're unsure how simple interpolation is done you can probably find a description with example calculations online. Then apply that simple interpolation theory to your 15 and 30 breakpoint tables which will be used for anything above 15.