You're right that all those issues can be sidestepped if you build projects in version controlled Git repos, test the code, and deploy JAR / Wheel files.
Speaking of testing, can you let me know if this PySpark testing fix worked for you ;) https://github.com/MrPowers/chispa/issues/6
Problem is that it’s limited. You can’t, for example, commit multiple files in one go (that improves a little bit in https://docs.databricks.com/projects.html), or merge changes a colleague made with your changes. You also have to use the UI Databricks provides. You can’t use a git CLI or whatever GUI you prefer. (all AFAIK, but I’m fairly certain about it)
My point is that you can do that even without jars/wheels - you can do VC and tests of notebooks. For example, https://github.com/alexott/databricks-nutter-projects-demo