I have successfully created widgets in Managed Workflow and released them on production! @wkozyra Thanks for your advice 
The development process was to create a Widget in Android and iOS as usual in the prebuilt (ejected) state and then create Config Plugins to make the same file structure. The process may be similar to provisioning with Ansible, etc.
For Android, it was relatively easy because all I had to do was prepare files. However, for iOS, it wasn’t easy because the “pbxproj” file needed to be updated (onesignal-expo-plugin was very helpful). The more files (swift files, fonts, localization files, etc.) used for widgets, the more difficult it becomes. It is hard but not impossible if the contents of the pbxproj file are steadily modified to be the same.
I created a minimal example repository for your reference.
It contains only minimum features. It will be more complicated when you create it. You might need Shared Groups, the ability to reflect app updates, etc. But who wants to create widget might refer.
Thanks,