Interesting project, though note that replacing existing system is extremely ambitious. For several reasons.
Maybe even too ambitious.
“Let’s reimplement <big project> in <something>” has many cases of failing, and only quite rare successes.
Well, complexities of MongoDB have own issues. For start it is not open source, what immediately makes it use in a core OpenStreetMap infrastructure a non starter.
As academic exercise - have you stress tested and compared performance of existing SQL setup with your MongoDB idea? Maybe replay of day of edits?
BTW, GitHub - Zaczero/openstreetmap-ng: 🚀 The Next Generation of OpenStreetMap — in Python! could benefit from making clear that it is a proof of concept / intended to be a proposed replacement / proposed replacement. Right now it is not really clear.
From use of MongoDB it seems to not be a proposed replacement?
In general suspect that improving existing codebase would be much better than “lets replace codebase, migrate storage, start using new database, setup new deployment and use new languages, add more features, change api, add edit filtering, everything at once”, as a viable progression.
For example: imagine planning theoretical deployment of that project, including ability to undo it and return to old setup, without data loss.
This alone would be a massive task to plan and prepare. And if at the same time many features are added it makes more complex to even test that something is backward-compatible.