We made a standie, the first trailer we built, the wife and I took it to quite a bit of places, we enjoyed it so we are no opposed to another one. The minimal requirements back then were to be able to stand up and put on clothes before heading out, minimal electrical system. Back then we also didn't have kids, now we have twin 3 year old girls.
Outside -
https://photos.app.goo.gl/CVtuwtqunhrcd6yE8Inside -
https://photos.app.goo.gl/T32LLjFyvMn59JKZAPros - It met our requirements, it was nice to be able to stand up and put your clothes on in the morning, plenty of storage under the bed, had a couch in the front of it which could sleep one, could have built a bunk bed in the same space. Storage under front couch too.
Cons - it was not offroad capable in any way with 13" rim wheels and even though it was not really heavy guess #1500, and the front was curved, I didn't get good gas mileage towing it. without towing anything I would get 17mpg, towing the standie, 13-14mpg, towing my 6500# boat 11-12mpg. It was tough to back down, the short camper and short tongue length were rough. The storage under the main bed was a pain to use lifting up the mattress to get at the hatch underneath.
My thoughts are to make something that was not taller than the roof of my truck, 2 track road capable. I am contemplating the pop top so as our bed would stay setup all the time, the pop-top would lift up and a set of army stretchers would be setup in the lift up portions for the girls to sleep in. In my first post I stated that I wanted to be able to put a tent camper on top, this updated idea is to skip the tent camper on top and integrate it into the camper.
I'm probably driving you crazy with my ideas going all over the place, I am in the planning stages and will be until next spring I am listening to all the points you are making and truly appreciate the advice and experience which is being provided.