The recommended solution to slow startup times has always been keep a repl open. That's basically the same workflow as a notebook in my mind. Like I said this means there is a large class of tasks julia a doesn't make sense for because paying the startup cost is too expensive compared to python or go.
I just timed vscode with the lsp. From the point I open a 40 line file of the lorenz attractor example, it takes 45 seconds until navigation within that same file works, and the lsp hogs 1 GB of memory. That's 5x the memory of clangd and 20x worse performance; hardly what I would consider a snappy experience.
I have no doubt that julia can be shoe-horned into realtime applications. But when I read threads like this [1], it's pretty clear that doing so amounts to a hack (e.g., people recommending that you somehow call all your functions to get them jited before the main loop actually starts). Even the mitigations you propose, i.e., pre-allocating everything, don't exploit any guarantees made by the language, so you're basically in cross-your-fingers and pray territory. I would never feel comfortable advocating for this in a commercial setting.
[1] https://discourse.julialang.org/t/julia-for-real-time-worrie...