I used a 7mm farmer john for years before switching to a drysuit. In many ways I think a farmer John is easier on and off than a one piece. Not sure at all where you got your info.
Definitely agree that the farmer john is super easy to put on. Basically put on some pants, then put on a jacket. You can get funny ones where the top includes some almost shorts and those would be a bit harder. In general though with the zips almost always at the front (rather than usually at the back) I have found 2 piece farmer johns easy to get into. They also give you a bit of versatility on the surface when you want to be wearing a wetsuit but not necessarily the whole thing i.e. When using it in place of a PFD on a small boat.
I have done a couple dives with a 1 piece + hooded vest. I near on dislocated my shoulder getting the vest off afterwards but I would say it was about as warm as my 2 piece farmer john.
If you go with this much neoprene be aware that it complicates your buoyancy. You are going to have a lot of Neoprene so you are going to need a lot of weight, this in turn entails a lot of air in your BCD at depth. That larger air bubble is going to swing your buoyancy faster and make things a bit harder on you in the shallows. This is why if I was purchasing a wetsuit again now I would probably have gone for the 7mm semi-dry with maybe a vest for winter if I really needed it.