I don't agree that brand-building is fine just for major brands, where they fight for small % of a saturated, well defined, market share.
Like you said, it's all about scale, and actions of a bootstrapped startup should adjust to it - not to be spending thousands on Youtube campaigns (picking up this post example). If you want some immediate useful action you should focus on media and placements where you have a higher percentage of people that a near to take immediate useful actions - maybe a video platform to consume video content is not the best choice to start with.
Brand building can be as simple as the way you address your customers, how your company looks, how the buying/service experience is, etc - things that change the perception of something random and generic, yet foundational to any brand.
McDonald's was the place to get tasty food without having to wait sitting down for it.
Where would that be for a candy subscription service? I mean, people looking to buy candy probably just want one packet right now, not ten spread somewhere over the next few months.
The most useful thing I noticed some startups do is get involved in their niche community and spent money building brand awareness for a targeted group.
I'm not sure that's true. Some people are just working on a different scale.
Brand-building is fine if you're Coke or BMW or McDonald's. You have millions to spend, you can saturate channels, and then people might actually think of you in preference to your competition when they're in the market for a drink or a car or a burger.
On the other hand, if you're a bootstrapped startup and worrying about affording this week's ramen, ttul's "tiny budget and short time horizon" is probably all you've got. To a first approximation, the only thing that matters for online ads in this environment is how many people you can get to take some immediate useful action, even if it's just volunteering an email address that you can use to follow up, or sharing your site with someone else they know who might be interested.
Of course, in the longer term, brand awareness is valuable as well. But branding isn't worth squat if your business won't be around long enough for someone to remember it later.