You and I don't have a meeting of the minds for what a founding engineer is. I understand it to be the first, non-cofounder, salaried engineer. This is the highest-equity receiving engineer-focused early employee. They should be getting a market-ish for a startup salary. In this day and age, in the US, it's probably 150-180k + 1% equity + healthcare.
If you are not making a salary / a well below market rate salary (50k for an engineer), you are a co-founder, not a co-founding engineer.
If you are not making a salary / a well below market rate salary (50k for an engineer), you are a co-founder, not a co-founding engineer.