Your pricing should be much higher in my opinion, something like $20-25/month, but then give those customers pretty massive access. (unlimited everything)
Doing the math here, for you to make a starter engineering salary off this project, you'd need to hit $10,000/month - which would mean around 40,000 paying basic users or 12,000 paying pro users.
If this hits the front page you'd maybe get around 50,000 hits to the site - which means your conversion needs to be around 80% (for Basic).
Whereas if you had a free plan and a pro plan that was $25/month - you'd only need 400 people. Which is a conversion of 8% from this particular traffic source.
Your call, but I think lower prices these days speak to me as the developer not being all-in on their particular product - even if you are, it's just the optics of it.
If I really need this, you can bet I'd pay $25/month for it.
The Roam Research guys do a good job at this, even if their product is not yet at 90% refinement, just the fact that they are willing to charge a higher price makes me believe they will actually put the work into it to make it better. There are students and researchers that do NOT want to bother with storing the files to build that graph, hooking up VSCode with a graph-editor addon (there are many), all they want is a thing that does what it says it does. And they pay well for it.