Having just gone through an evaluation for platforms just like Nextjournal, there are a lot of companies that make similar claims, but very few that deliver in reality.
In the end, the only one we found that delivered on the promises of reproducibility and managing the entire data science life cycle end to end, facilitating collaboration, and getting stuff done was Domino Datalab[1].
Can you compare and contrast Nextjournal to DD? Better yet, do you feel you're competing in the same areas or are you really more focused just on reproducibility? Even if you're not now, it feels like eventually, all these types of products seem to converge to this state eventually just by nature of the sales process and promising more and more features to customers.
Regardless, it looks really solid, so best of luck!