There are plenty of minor challenges getting PyQt5 and Qt Designer to play nice with PyCharm, but after getting all the small steps in place, I cannot help but wonder if I missed the obvious.
What is the most straightforward way to integrate PyCharm and Qt Designer?
What I did so far:

Install Qt Designer
Set it up as an external tool


Open Settings > Tools > External tools
Add a new tool
Set the Arguments as $FilePath$ and the Working directory as $Projectpath$

Rightclick .ui files in the project explorer and launch Qt Designer from there
Set up a File Watcher from Settings, watching for changes to Qt UI Designer Forms and running pyuic5 with the right arguments to generate the matching .py for my .ui

Answers I'm looking for:

How can you tighten the loop between Qt Designer and PyCharm? Specifically, can the Qt Designer be opened on a simple double-click from PyCharm or even in a tab in PyCharm?
Is there a better overall workflow that achieves the same, that I'm missing here?
If you are just looking to open the .ui files in QT Designer, there is a simpler solution. 
Go to Settings|File Types and click on "Files Opened in Associated Applications", the go to the Registered Patterns field and add *.ui as a pattern.  It will complain that *.ui is already registered to QT Designer. Click OK to reassign the wildcard. Now, when you doubleclick on the .ui file in PyCharm it will open with the associated editor in Windows (which should be Designer).
If PyCharm has already associated the .ui extension with some file type, you can easily override that by selecting the file in the Project browser and selecting File|Associate with File Type... from the menu. Select Open matching files in associated application to have PyCharm open whatever application has been associated with the file type in Windows.
Integrate Qt Designer and PyCharm