Help me - whats my next step?

[Nisshind]

So i'm gonna start this off with - I'm a developer - i know very little knowledge about SEO&Marketing, but I'm trying to figure out how to proceed my side-project - so thats why I'm here asking.

1½ year ago, i created a internal site (in my free time) for my old work, that gathers expired (mostly nordic) domains - and fetcher relevant SEO-data. But around 6 months ago i got a new job - so my site was mostly inactive except for personal usage. A couple of months ago, i decided for fun to remove the password, and let everybody have access to it.
I quickly saw a couple of visitors, and people seems to like/use it (Google Console says avg. time spend on site is like 11-12 min - when coming from google searches). So i decide to overhaul the whole site, improve performance, move it to a .com domain, and add some more functionality - so thedomainrobot.com was born.

But I'm now at a point, where i cant find any more stuff to code - so I figured, I should working on something i have been trying to avoid - SEO&Marketing. But I'm totally lost, where should i start? what should i do? do i need to throw in some money at my "problem", to give it a boost? should i buy guest blog-posts?

My best idea so far is working on some domain related blog posts - but is that enough? i would love to see if i really could get this site going, and build up a decent userbase - and learn some SEO&Marketing stuff at the same time