You've spoken well. I've noticed first hand how images can break a content and it really hurts to see how one's efforts go down the drain because of their choice of images and the source.
For Canva Pro, I've subscribed to it at some point when I really needed it to get things done. It safe as I have experienced first hand. I didn't notice any unwarranted charges, etc. But you may still need to go about it with due deligence.
For the aspect ratio, 16 by 9 has remained the sweet spot as most devices can easily render it. For those who don't know, here is what a 16:9 (horizontal) image should look like:
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16:9 is the best like we both agree on. The vertical one is preferable if the content is probably not a proper blog post. For example there are some posts where I use 16:9 for the first image and this is because I want the image of the post to appear well, even without clicking the link on the post. So when I create my own images I have the ability to tweak it to my taste. This is probably one of the disadvantages of people who use stock images.
Moreover, I like the originality and the different feel of creating one's own image.
Thanks for dropping those sample images, you could naturally say that the first one looks rather attractive.