Looking at the heading of the question it may seem like a duplicate question, but it is not. Let me explain a few things before anyone downvote or make it a duplicate.

As far as I know in WordPress the only way to make custom image size is by using the add_image_size(). Now let's say I have a CPT called Testimonials and at the time of registering the CPT I have added thumbnail support.

Then in my functions.php if I have like the following:

<?php
add_action( 'after_setup_theme', function() {
    // Custom Image Sizes
    add_image_size( "my-testimonial", 100, 100, true);
});

Then in my template while loading the thumbnail of the CPT I can do this:

the_post_thumbnail('my-testimonial');

And this will give me a 100x100 image as the thumbnail.

Problem With The Above Approach

Though the above approach works fine, it has a major downside. As add_image_size() is global, after this point any image I upload, whether that be for a blog post, a page, this CPT or any other CPT, each of those image upload will have a 100x100 version in my server which is simply wasting both server execution & storage resource.

Moreover, if I use any image optimization API, they are going to optimize all the images regardless of whether I use them or not and charge me accordingly.

What I am Looking For

So, what I am looking for is a way to specify the thumbnail size of specific CPT so that if I upload any featured image for that CPT then only those images will be sized to what I want but it will not crop any other image outside of that scope.

I have Googled a lot but couldn't find any solution to this dilemma. Moreover, most of the answers are old and now WP 5.4 has been released, so I was wondering if there is any way to accomplish this now.