There are no floor joists, it’s a concrete slab foundation

We don't have hardwood but large plank tile and we ran it going with the longest stretch of the house and hallways. It doesn't run straight from the front door. I'd do what makes the most sense.

If you have concrete floors aren't you better off using engineered hardwood flooring for better stability?

Yes, engineered 👍

Ok. I find it better to lay it the long way in the public rooms and let the other rooms fall how they may.

On our main floor the engineered wood runs the length of the house left to right. So when you walk in it is left to right in front of you. For our main floor this makes most sense.

Downstairs it is actually LVP but it runs the other direction toward the back door but again makes sense down there.

The main living space will dictate the direction of the flooring. Think of the living room/great room as the 'public' space that visitors will see. You will want to impress your visitors. That means you want the MAIN great room to be set up properly. Figure out the direction in the great room and then move on from there.

Although a single direction is ideal, it doesn't always work. It is possible to run the hallway in one direction and then have decorative transitions to allow the main rooms to have the hardwood to run the other way. This can be more expensive and take a bit more time for the installer, but it can be VERY handsome.

Bedrooms are allowed to change direction at the threshold of each door. Again, an attractive transition is a nice touch and adds value to the floor and to the home.

Adding to SJ McCarthy's statements, by having some decorative transitions, it also allows you to isolate any future repairs or refinishing to a certain room/area.

My current site finished wood floors all run together in my downstairs and it has been a problem when we've had to do some repairs, like when we moved stuff around in the kitchen remodel. It meant we had to re-coat the whole downstairs instead of just one area/room, which involved getting movers to move everything to the garage for 2 weeks. Not fun.

I plan to do some decorative transition in the future house to provide some transitions in the open floor plan that will allow some separation for refinishing or re-coating.

We ran my wood flooring the length of my house. Looks fine.

When walking into the front door, the longest part of the board should go from left to right.

Personally, I think transition strips disrupt the flow of a house in most cases.

"You will want to impress your visitors."

That is just so sad and wrong.

We did a transition between the kitchen and dining/living room with engineered hardwood. It is completely flat and allowed us to lay the floors in the longest direction for each room. here's a pic.

Here's a good article that shows some good ways to lay hardwood flooring. My personal favorite is the Herringbone design