I am going to float engineered wood over concrete on the first floor of our house. I will use 6 mil plastic under 1/4 inch cork under the 3/4 inch wood. I will glue all sides, leave the expansion gaps at walls and all "fixed" locations.
I am attaching a diagram of the floor. The shaded parts will not be done.
I will run lengthwise from to back of the house. The longest stretch is 53 feet, from front door to rear exit. the widest is 39 feet from family room to far side of kitchen.
My questions:
Where should I start? I do not really want to start in the middle of the floor, but can if that offers great enough advantages.
Must I put in expansion gaps here and there throughout the floor? I can see putting them in doorways, but do I need to put them anywhere else?

Any other guidance you can offer would be much appreciated.
Here is a link to the floor diagram.
https://i.imgur.com/LSYA7qf.jpg

Starting in the center of the house will be your best option to account for seasonal changes in wood dimensions. Acclimate for the most moisture in the air that you will have in your area. Floating floors will move more than nail down. Can you get 3/4 engineered flooring? This will be better than solid. You will not need to have the expansion gaps at doorways.

How do I start straight and stay straight?
I will leave expansion gaps where the floor meets and vertical item like a wall, but do I need expansion gaps mid-floor?

Use a chalk line to make a straight line. If you like, you can use a pencil and a straight-edge to darken the line. The same straight-edge that you used to see if the floor is flat to within a quarter inch in ten feet, or an eighth in six feet.
You will not need expansion gaps in the middle of the floor, just for obstructions.
We used to scribe fit between the door jambs, but now it's undercut the door jambs, along with the casings, if you are working after the trim is installed.

Pete, excellent guidance, thank you. I will post pics when I am done so I can show you what you help me accomplish. If I have questions between now and then I will come back.
If I start in the middle as you suggest, is there any reason I cannot work backward on the one side and avoid a spline?

Spline is good when flooring is back to back. You don't need it to keep the boards aligned if both boards are glued down. You can make spline from quarter inch hard board and a block plane to ease the edges of the spline.

Thank you again. I will post pics along the way and keep asking questions, if you have other guidance I am appreciative.