Thank you for the extensive feedback! As I see, there is no perfect solution so far... I had to deal with git submodules in one of the previous projects and remember the experience was not that nice. Also tried to publish them to NPM in another project, it also comes with annoying workflow in a fast-changing codebase. So I have to really think about the way for the next project...
I consider the mono-repo approach to provide the least friction when it comes to fast changing codebases. Throw in a CI solution, automated tests+typescript and you end up with a very easy to maintain workflow.
This is especially true for full-stack TS projects, where sharing types between the client+server-side code can create an extremely "refactor-able" code base.
I also should of mentioned you could use dedicated mono-repo tools like lerna even for smaller full-stack projects to help share code. Generally though, if you package code (using npm or other systems), you have to deal with versioning pains.
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