Saving complex artwork as picture file then re importing into Ai

Dear Adobe Community, I've been using Illustrator to produce large scale technical illustrations. Most of these are highly complex over muliple layers. To make the life simpler for me and the producion, I like to save the finished illustration as a jpg in 100% scale/900dpi and then re inport the image into a new illustrator file of the same dimentions and then add the text, graphics etc around. Doing it this allows me to move or rotate the image/illustration to into final position far simpler than the multi layered option. I then export the file as a PDF to be sent for production on super high quality digital plotters

My questions to the experts is:

What would be the best file type to save the original illustion?

what would be the optimal resolution?

What would be the optimal pdf settings for the final production files?

Is this a good way or is there a better/more correct way of doing this?

I have been using Ai since 1988 and still haven't got any idea how it works or the correct way to do things!

The technical suff simply isn't my thing.

You can see the type of work a do at www.davidbathe.com

This is a genuine request for those knowlegable in such matters to help me out here.

Peace and love, David.