However, as a new user it's not immediately clear where to click, for example the button in the top left corner which lets you switch between the plan and the diagram editor. Also the small sidebar for inputting the data might not be the most optimal place to do the data input.

What is the goal of the application? What is the benefit of having the diagram? Maybe I'm misunderstanding the purpose but to me it seems like the main output from this software is the "maintenance table", but how do the pointers/diagram help instead of just having a dropdown in the table?

Maybe you can share a more elaborate example and then I understand it better.

Overall, could be more polished but I really like the simplicity and the combination of table & visualization.

Another thing. Regarding the field "effort" it is measured in days, it should IMO be measured in hours, or even better yet be user configurable.

You might want to include a screenshot of a real-looking plan to give people an idea.