Google just changed their rules for GMB posts: If you are a chain, you can now mass post to all your Google My Business locations using a GMB post scheduling tool.
Source: Create Posts on Google | Google My Business API | Google Developers
From link above:
Source: Create Posts on Google | Google My Business API | Google Developers
From link above:
Note: In response to coronavirus (COVID-19), chains may now create Posts via the API for a limited time. Posts made by Chains should be related to business updates relevant to coronavirus (COVID-19).