Backlinks without an ad budget, does it actually work, or is it a myth for bootstrapped founders?

Been submitting to the usual directories (AlternativeTo, StartupRanking, etc.) and just had a good discussion in another thread about how a lot of the paid "guaranteed top 3" directory badges are nofollow anyway so you clear their conditions and still get zero SEO value out of it.

No paid ads here, so backlinks have to come from somewhere real: guest posts, being mentioned in a roundup, someone linking to a tool because it was actually useful to them.

For those of you doing this on a bootstrapped budget what's actually worked?

• Directories that gave you a real dofollow link worth the submission time?

• Guest posting / outreach that didn't take forever to land?

• Anything that surprised you a link you didn't expect that ended up mattering?

Not looking for "write good content and it'll happen" curious about specific tactics that actually moved the needle for you.