Asynchronous control framework
I recommend writing your own in C++.  Arduino works well with C++ as it is written in C.  You can focus on getting the asynchronous behavior to do what you need instead of writing drivers.  I don't think it will take any longer to learn enough C++ to make this work than it will to learn to use, and possibly fix, a library. Your C++ knowledge will be much more generally useful than knowing a specific library.

Igor points out the advantages of going to a real time operating system.  I agree and think that, if you want to work on robots long term, you should go with an RTOS and get away from Arduino. Working with C++ and Arduino is one way to get started.