Hi,
I have a vision of myself doing most of my scoping in a seated position, so the tripod would be extended to mid-height. It would be possible to move the extender center post up and down. I've never used a tripod before so I don't know much. My gut instinct still tells me that a small straight scope would be suitable. But I'm guessing. Also, is it possible to use the ED50 x16 fixed hand-held?
I think it would be much more convenient to use an angled scope when sitting in front of a tripod.
Not only does the angle help you to look up more easily, but it's also much easier to adjust one's head position to a new viewing angle than with a straight scope, where you'd have to re-adjust extension heigh with each change.
With regard to hand-held use, I have both a straight and an angled body for my ED50. I thought the straight one with the stay-on case with hand loop would be great for viewing shorebirds at long ranges, but I have to admit that the combination of monocular view, and large magnification with resulting enlarged shake makes it no more effective at 13x than my 8 x 42 binoculars.
(In your application where a wide field of view is beneficial, that would be another point in favour of using binoculars instead of the scope.)
I did however 3D-print a shoulder stock to use with the ED50, and that helped a lot with steadiness. Here's what it looks like:
https://www.thingiverse.com/thing:2807320
While the picture shows the straight version, I found that the angled version is even more convenient to use with the shoulder stock for some reason.
So I'd think that for your primary application of using the scope from a tripod, angled probably would be better. For the secondary application, using it hand-held, straight might seem like a good idea, but in my opinion it only makes sense with a shoulder-stock, and then angled is as good or better.
The main reason to pick a straight scope is usually if you're birding out of a car window. I've also heard one birder saying that he likes it better when looking down from observation towers, which makes sense too.
By the way, you can also mount binoculars on a tripod - the image really gains a lot from steadiness. That might be a way of trying out your original idea and the ergonomics of "straight" optics.
The same applies to the shoulder stock - the 3D print I linked above also works with binoculars, and you'll find an image of such a setup if you click through the pictures. It works really well, but I'll admit that it's a bit inconvenient to carry around the neck
With regard to the eyepiece, I have the 13-30 zoom. I would agree with Dalat that for your application, a fixed eyepiece with a larger field of view would be the better choice.
Regards,
Henning