hello Alan, I totally agree about the 100-400 and in fact If I get that I will strictly combine it with 5DsR so as to use it for landscapes, flowers in addition to birds. In that case I would have room in my bag for 5D4 with a 16-35 lens. Somehow I never had huge success with birds and 5DIV (at least with a big lens and teleconverters) although it is fantastic for low light photography (and with my 500II for some specific shots on a tripod). The excursion will cover a broad areas like a boat, walking on a hill to watch the area, in a forest, close to a small river. etc.Currently away on a 3-day trip to the Farne Islands on a Puffin photographic trip. We have taken both the 5DSR + 100-400mm II (which my wife monopolises) and the 5DIV + 400mm DO II for me, ok the 5DIV is not the 7DII, but there are some parallels. The 5DSR wins for static shots and slow BIF. But, the 5DIV has been fantastic for the Puffins and other small birds flying like rockets. The 5DSR focuses pretty well on the central 9 points but the 5DIV is faster and works on the lot. The 7DII doesn't focus as fast as the 5DIV but has close to the reach of the 5DSR. So, if you are mainly on the static stuff, I would go for the 5DSR because of its wider fov and better IQ, and I would take the zoom because of its greater versatility and shorter mfd. But, if you are into flying Puffins etc, it's the 7DII.
Edit: Oops Tron, you posted your mail while I was writing my response, which is now partly redundant. The bare 100-400mm II is pretty close to thje 400mm DO II in terms of IQ, and very fast AF.
Do you still use teleconverters with the 400DO II on your 5DIV?