I don’t think it ticks every box for you, but have you tried Jupyter Notebooks in VSCode[0]?
[0] https://code.visualstudio.com/docs/datascience/jupyter-noteb...
[0] https://scalameta.org/metals/docs/editors/vscode/#worksheets
Again, not exactly the same as Hydrogen but there’s definitely some overlap in use cases and VSCode’s Jupyter Notebooks is pretty good. You should give it a shot and see if it’s a viable replacement for what you were using Hydrogen for.
This showcases the UI a bit if I’m not being clear: https://code.visualstudio.com/learn/educators/notebooks
As I noted in a sibling comment, I wrote a quick extension [0] that makes this easier by automatically inferring code blocks, but VSCode Jupyter is lacking in other ways: you can't have multiple Jupyter kernels running at the same time and it doesn't show you runtime completions in the editor (necessary for compiled packages without type support).
I'm giving VS Code + its Jupyter extension a shot for now. It's significantly worse than Hydrogen, but I tried to make it a little less bad with a quick extension to auto-infer code blocks [0].
It's breathed life into my love for programming, given how easy it is to just start executing code (sometimes I just use it as a calculator app since it's that easy and accessible)
I tried VSCode's equivalent plugin, but the UX was nowhere near as nice.
Multiple concurrent kernels, the ability to connect any .py file to any running kernel, sharing variables and imports across multple .py files, inline matplotlib output that persists on-screen and inline with the code even when running other code cells... the list goes on. I'm really fond of this setup. I use it daily--it's an essential component of my day job--and intend to do so for the foreseeable future. VS Code does seem to be the way of the future but there have just been too many friction points for me to leave Atom.