Workflow for re-using data structures and setups across projects? I am wondering if there is a way to re-use pre defined field types and image crops over various Craft projects. I currently in the process of e.g. manually defining 20 different image crops I already defined for another project, which is a tedious task.

I am not really familiar with SQL and database structures, so I am not sure if there is a way to simply copy fields from one projects database to another or if this would corrupt my database?
You have several options to solve this issue.

Import fields via plugin

You could create a plugin that handles all these function in it's installation callback so you would install the plugin and have all the things you need. But this requires basic knowledge in plugin development. 

Advantage When Pixel and Tonic make any changes in their install process it won't infect your code and if you are going to make multiple plugins you'll be able to import the required fields whenever you use them. That's the way we handle it and when someone installs my Form Builder Plugin it includes a form relation field to each entries with type pages and some crop rules automatically. 

Disadvantage You need to learn php

Import via SQL

Install a new Craft instance, insert all your default values you would like to have for a new project and export the entire database. When you start a new project you would skip the installation process but import your included database and the files of your "default" instance.

Advantage This is an easy and really fast way to achieve your goal. It does not require any knowledge or skills

Disadvantage You should pay attention when Pixel and Tonic makes core changes in their default structure or things could eventually break in the future (even if it is really unlikely because they are really careful with those things)

Search for a plugin from someone else that handles it

I know there is a Feed Me Plugin maybe this does what you want but I never really used plugins that are not created in my company so I can't tell something about it