Featured Images

Ideally you should upload images of all the same size and then theres no need for CSS or plugins or head banging :)

A simple option would be this CSS:

.single-post .featured-image.page-header-image-single img {
    aspect-ratio: 16/9;
}

As it defines the aspect ratio as opposed to defining any fixed heights, all images will ultimately fill that space.
Adjust the ratio to whatever meets your needs.

I'll try that, Dave.
Thanks.

Is that Simple Custom CSS or Additional CSS?

Either one of the Simple Custom CSS or Additional CSS will do.

You may want to consider choosing one of the two places and putting all your CSS in there.

Oh, ok, I'll put them all in SC CSS.

Thanks, Dave !

You're welcome