These applications are not identical and are designed to work together, not replace each other.
You have all the options. Pick the best one.
I know they are designed to work together - not to replace each other. BUT, if I can't migrate from one to another then I don't know how are they working "together" Again, I am aware that I would have to do some manual labour but my main concern is that I apparently have to start from scratch! I don't mind that there is no "magic" button that does what I want, but to redo all the text is not "working together" I "know" why I cant copy/paste from AI to ID (different text engine) but at least some useful import/export tools would be appreciated!
There already are third party tools to convert PDF to ID. You mentioned in your original post.
What’s lost in all of this is WHY you want to do this. Perhaps if you back up to that we might have something to help.
You got all the info you need in your other thread in the Illustrator forum.
You don't have to re-type your text, but you will have to reformat it. Copy it by selecting with the text tool, and it will come across as live text.
If you set up Paragraph Styles and Character styles before you begin, reapplying the formatting could be relatively painless.
As to the rest of it: any logos ad diagrams created in Illustrator can be put onto separate dartboards, and then placed into the InDesign document.
Photos can be directly placed into InDesign.
For speed of redoing the layouts, you can place the entire page on a locked layer in the background and use it as a template, them delete the layer when you are done.
Sometimes you can use a CC library to exchange the content, try that.