How to mass-delete inline styles in WordPress custom post type automatically?

I would use the Search Regex plugin. It will allow you to use a regular expression to find and replace (in your case delete) inline styles. Make sure you backup your database before you begin!

A simple regular expression that should work for you would be

(style=").*"

It will look for a string starting with style=" and containing any number of characters until it hits another double quote ". (That would be an inline style applied to an HTML element). You’ll “replace” it with an empty value (to delete it).

You can also test your content / regular expression with Regexr to ensure the one I gave you will work.

Backup your database before you begin. If something goes wrong you can restore it. It looks like this plugin will allow you to Search (as a test) before you run a Replace & Save.