Best resources to learn how to market your startup?

Hi,
I'm the founder of buust.io -- an online mentorship platform for beginner web developers. Given it is a dual-sided marketplace, I knew things would be difficult to market. I have enough mentors on the platform (for now), where I am confident that my main issue is not attracting enough beginner webdevs interested in mentorship.

I did customer discovery and this seemed like a problem without a quality solution, but I am honestly at a loss. I have been doing one-on-one cold outreach on Reddit and Twitter, growing my SEO blog, and even doing some paid advertising.

Are there any online resources out there that helped you guys learn marketing / how to grow your startup? Even on IndieHackers, I feel like there aren't many resources specifically regarding this. I see things like productlaunch.io, but other than that -- any suggestions?

Thanks so much!