We also definitely want to open source parts of our product but we haven't figured out what parts (or everything) and under what license.
Our priority is currently on providing a useful hosted product and become sustainable. It's certainly also interesting to see how e.g. metabase is doing it the other way around, open source first without a hosted product but I guess I'm a bit scared of not being ready for developing Nextjournal in the open at this point in terms of bandwidth and keeping things backwards compatible.
From my past experiences there are a lot of enterprises that are rightfully scared to let their employees use such a service and open up data regardless of what promises a SaaS company makes. There is general assumption that if the SaaS company fucks up, all we get is a "we take our security very seriously...." blog post.
So, making your product work within a corporate network without "call home" is a great advantage and immediately expands your target audience with some potentially big pockets.
You might want to check out VizierDB (a project my group is working on). It's a self-hosted multi-language notebook with versioning, branching, and snapshots; as well as a spreadsheet-like editor and provenance-based data annotations.
So, Jupyterhub and manual tinkering to get such polish for now.