How to change text format of summary/trimmed field? Is there a straightforward way to change the summary/trimmed text to "full html" in Drupal 8?

There are workarounds such as creating separate body fields and using them as the summary but I am specifically asking about changing the text format of default summary/trimmed field of "Text (formatted, long, with summary)".

To put it another way, how to I add the CKEditor bar to the summary/trimmed text.
You can't, it always has the same format. The field only stores a single format. And it has bugs with ckeditor, as you've found.

I would definitely recommend to use two fields. We'd like to remove the text with summary field type, it's unnecessary complexity but it was too late for 8.x.