Besides the prison cell gambit, it also helps lenny and myself that we've been campers all of our lives, so we know "exactly" what you need to get along and do it well.
In regards the floors, I imagine the ribs are there to make it easier to slide heavy cargo in and out. Also, as mentioned, the thicker you make the floors, eg adding boards on top the ribs, means that much less headroom.
I have enough floor space that I can erect my folding chair inside, and still have 3'x3' to move around. I've not seen any of the youtuber builds have enough space for that. I originally had the mattress lengthwise in the van and I only had 3'x'3' of open floor space, and that was horrible. I would tilt the mattress upwards to the side, but because the van is not so tall, it would stick out into normal headroom space, so it wasn't very ideal. You can cut a foam mattress down to 30" wide, using just a carving knife, and then it will flip upwards to the side, but having the mattress crosswise in the back is 10X better.
I do plan to redo the solar electrical business to remove the cabinet and put the panel on the wall, and which will give me more open floor space, almost 5'x6'.