Adobe Illustrator has an isometric feature (Effect > 3D) that is good for creating anything very basic and straightforward. However, in order to have a lot more control over what you create, it is best to automate what you can and do the rest manually. Here is the process I have figured out for myself while creating an isometric illustration of a ship for my next coloring book. I will share the ship illustration in the future when my book is done, but for now let us take an example of creating a simple cube with rounded corners.
Note: You can automate step 3 by recording and saving the steps for each of the three sides as actions. For the top side, here are the steps:
Open the Actions panel (Windows > Actions)
Select the object for the top side
Go to the Actions panel and click on the “Create New Action” icon
Give your action a name (Isometric Top) and click on the “Record” button
Perform the scale, shear, and rotate transformations on the selected object
Go to the Actions panel and click on the “Stop Record” icon
The next time you want to create the isometric top view of an object, first select the object, then select the Isometric Top action, and then hit on the Play icon.