I've looking this solution for hours! The v3 is way to old and buggy. Anyone has been able to upgrade it to v4.3?
I have written a module which upgrades the editor to TinyMCE 4 and also covers the customized plugins (widgets, variables, images), currently working for Magento 2.1 and 2.2.
For those who want to check it out: https://github.com/pavelleonidov/magento2-tinymce4