Imo that’s not a good idea, from a mere fact that most readers dont bother reading after 500 words and I’m being conservative there. 1500 would be max.

The problem with combining all of those articles together is that the topic or content has a lot of keywords but dont specialise in any. It’s better to break off several pieces of content from it and have each of them specialise in one keyword. And then as you said have a pillar content and link it to the main content.

This way you have more changes of increasing and showing up on various topics than relying on your pillar content for it.

For eg:
Cars as the pillar content
Then break off into hybrid, electric, etc .
(Just a silly example)

That’s my few cents as a person who worked in content. Nobody wants to read 7500 words on a webpage.

Maybe it would work better again as a pdf book (not kindle) but it needs to have evidential data and provide information as a way to create leads. Eg: social media platform, hubspot have these informative reports that are downloadable but they are always embedded in a article summarizing it.

All of this depends on your niche as well.