From my reading (not experience), a splice or scarf joint is the appropriate way to extend lumber with a good amount of strength. There are many variations of these joints, some which are more elaborate and have locking keys. Others are simpler and are not much more than a sloping half-lap, where the slope is 6-8 times the thickness of the board. Strength is supposedly far superior to a simple half-lap.

I would really love to see someone using this joint. I think they are largely used in carpentry and boat building, but I do not remember seeing one used in furniture. I suppose machined finger joints have largely replaced the scarf joint these days.

great video thanks Scott

Don’t rule out creating the table top as 3 foot sections with contrasting dividing wood. Sometimes its easier to celebrate a joint than to hide it.

What you are looking to do has been done many times before. I would construct it like a very long frame and panel door with more than one cross-members. You could leave off the stiles if you wish. Making the panel pieces flush with the frame would be the way to go.

I was recently in an indigenous village in the amazon river basin. They used a lot of the scarf/splice joints. They had very limited tools and an abundance of dense wood (we call that exotic wood). The joints were used in the rafters and even the ridge pole as well as benches.