Neat result. I've sold a few side projects over the years, it's always nice at first, though I've then ended up wondering "what if" as the years go by :-)

This jumped out to me:

We settled on an amount equivalent to how much I’d earn as a US-based engineer working 6 months, but then multiplied by X for the traction/success factor already achieved.

It sounds like you have a pretty significant win here, considering there was no real revenue. I'm guessing we're talking at least $50k multiplied by something.. so not exactly pocket change :-)