Perhaps this is possible and I've missed it, but I'd love it if there was a way to set the default stack thresholds in Mod Settings, so that when first placed the combinator would be set with a predetermined value. This would be like what is currently possible with flat thresholds. (e.g. set provider stack threshold default to 40, and flat threshold to 25000)

The stack and "flat" thresholds are LTN signals. This combinator just gives you a comfortable UI to modify them per station. The ability to set the default "flat" threshold is also an LTN setting, not an LTN Combinator Modernized setting. I assume the reason there is no default stack threshold is because because if as stack threshold is set, the "flat" threshold is ignored completely.

For changes, or improvements to how LTN works internally you may want to make this request at: https://mods.factorio.com/mod/LogisticTrainNetwork