As someone who's new to the ebook world, I totally get your concerns about having lots of images in your book and how it might affect the size of the MOBI format. Right now, you're saving it as a Docx file, which is a good start. But you're worried about the file size once it's in MOBI format for Amazon.
You mentioned that Amazon provides a tool called Kindle Textbook Creator for image-heavy books, but you're wondering if there are any other impacts or if anyone has used it before. Honestly, I haven't personally used that software, but it does have some cool features. However, keep in mind that it might limit your book's availability to only Kindle devices and apps. So, if your target audience primarily uses Kindles, it might not be a big deal.
If you're looking for a more versatile option that works across different ebook platforms, you could consider converting your book to EPUB format instead. EPUB is widely supported and gives you more flexibility with device compatibility. There are handy tools out there like Calibre or Sigil that can help you with the conversion process. They'll also help you optimize your images for efficient file size while keeping them visually appealing.
Another trick to shrink your image-heavy book's size is to compress your images. You don't want huge file sizes slowing down the loading time. Tools like Adobe Photoshop or online services like TinyPNG can help you compress your images without sacrificing too much quality. I did this for my book series "The Little Witch Academy" as well.
Good luck with your ebook publishing journey! And hey, if you want more info, check out littlewitchacademybooks.com—they've got some helpful resources too.