These are what I would consider high-effort, low-reward marketing strategies.

It takes a lot of time and manual effort to identify communities and pages where this activity should take place, and then to initiate the activity in a way that doesn't get flagged as automated/spammy. For your troubles, you may luck out and get some new followers who are mildly interested in your product, or some clicks that probably won't have high conversion rates.

They may be "powerful" in terms of getting views or initial interest, but they likely won't be powerful enough to get people further down the funnel.

Do you have alternative strategies for one-man businesses with 0$ budget?

That's neither "uncommon" nor "powerful".

Most of the these "strategies" lie pretty much on the surface. Also, half of them amounts to blatant comment/blog spamming, basically. Ditto with Reddit - it may work once, but try sneaking your promo there under a disguise of a genuine RFC again and it'll get you promptly flagged and banned.

I'll make it simpler: participate in whatever community you're in.

Off-medium: https://outline.com/4RYJ8T

This is a cool app. I can't seem to find any more info about it. Does it have an API? Browser plugin? Open source?

You can see some info on their frontpage https://outline.com/

There are some links at the bottom.

As far as I can tell it seems proprietary software, couldn't find the source or anything about it being opensource anywhere.

ok. I have now signed up to Instagram. Is there a limit of how many posts I can like? Can I get shadowbanned? Should I only like relevant stuff?

There are some limits, but as a human user you likely won't hit it. They are good at catching bot traffic.