Where do you invest most of your resources after the product is launched?

It actually depends if you reach product market fit or not. If you still have a low activation rate, you do not want to invest in user acquisition yet, you should rather focus on improving your product and onboarding flow. If you need extra users while working on product-market fit, growth hacking will be my go-to channel as it will be the cheapest.

User Acquisition (not paid media until a few months down the road) primarily for what we've done, but as others have mentioned it varies wildly based on goal / what phase the product is in, etc. We like to dial in social channels for organic reach as early as possible, so that there is a baseline level of content already there for the curious folks, and other than that its strictly user growth until we can justify SEO + PR

I think if you ask regards the "product validation" stage, then I would suggest social media, for quick results. IF that picks up, most likely (depends on product) rest of the channels. Good luck!