Ethernet is rated to about 1,000’ - Cat5e should get you 100MB while Cat6 should get you to 1GB. Fiber is a nice backbone option, you’ll just need to take special care in running it and you’ll need special switching hardware at each location.
You will also need to map out the infrastructure. Phone and data will likely be seperate services and therefor require seperate networks. If each house is going to have a unique phone number, the phone company will land the service at a box at each house, the builder should provide a location for them.
As for simplicity - I am always amused when a customer/ home owner/ builder starts the conversation with, “we want it simple” and then proceed to outline a system like the one you are describing. There is really nothing simple about it. You have not “gone to the dark side” but your builder/ project manager is in the twighlight zone if they expect simple or inexpensive (usually what is meant by simple).
Our job, as system integrators and custom installers is to do the planning and thinking - hopefully the end result is that the customer thinks of it as simple while appreciating the effort and work required on our part to make it so.
So now you get the enjoyable task of clearly defining the scope of work from a project that sounds already a little out of control. Nail down your commitment even if they keep moving the target. Write it down, get a signed contract, bill them promptly.