Opinion on Sliders, Carousels and the Hero image.

"The hero image seems like a more SEO friendly choice."

I don't know what you've been reading but SEO is about text content, not images.  Google doesn't care what kind of eye candy you use.  Carousels, slideshows, hero images or itty bitty thumbnails are all just images. 

Google bots are spidering text all over the web in search of unique and relevant content inside good semantic markup.  Read Google's SEO Starter Guide for pointers.

https://developers.google.com/search/docs/beginner/seo-starter-guide

As osgood said, bandwidth is another issue.  Pages containing too much bandwidth won't load quickly and may perform badly on slow data plans.  No matter which type of eye candy you use, images should be fully optimized for the web in Photoshop or Lightroom before you bring them into your site.

Doesnt really have anything to do with SEO at all. Its' more to do with bandwidth. Obviously band-width is more 'valuable' on mobile devices if you are paying for it as you browse, whereas on 'static' desktop devices it doesn't really matter, most users will have packages which provide unlimited bandwidth.

Probably a good solution/compromise would be to deal a slider to desktop devices and just a static hero image to mobile devices, if you can be bothered. A lot of developers including myself can't be bothered so usually both devices get a slider as its visually more interesting than a static image. (Although I will usually try and make an effort to reduce the size of the slider images for mobile devices).