Paid acquisition gets a startup its first customers and then quietly eats the margin. Organic search is the channel that gets cheaper the longer you run it - but only if the foundations were laid early and the effort went into the searches that produce buyers, not the vanity terms that produce screenshots for the board deck.
The reason why SEO for startups is different from SEO for an established business comes down to sequencing and constraint. You can't do everything, so the strategy has to pick the handful of searches that map to your best customers, build the technical foundation once so it scales with the product, and create a content engine that the team can actually sustain. That's what I do - senior-led, no agency layer, and honest about what's worth doing at your stage and what isn't.