I had the same set up - I bought a cheap treadmill off Amazon:
As far as treadmills go it is fairly light and fairly manoeuvrable - but it’s still a pain to put in place then pack away, so I used it for about a month and now it is just stored away. I think I might have put up with it but I find walking on treadmills for long periods make my hips ache, and I think this is because when you’re using them it’s an un-natural movement, and that’s on top of what you said yourself about it making it awkward to type so it just didn’t feel worth it.
What I do like tho is using an exercise bike underneath my standing desk - far easier to type and much more enjoyable as you can stop cycling as and when you need to. I’ve got one of these:
I also tried a magnetic one because the one above is not ‘silent’, however I much prefer the one above because you can actually get a great workout with it when you turn the resistance up (so great for Tabata style sessions - though obviously not while under your desk).
I also use a kneeling stool so it’s easy to alternate between them. I haven’t used my bike this year but will get it back out soon.
Another thing I do while on the topic, is periodically grab a mace when I feel like I’ve been at the desk too long, one of these:
I considered one of these but I’m a germaphobe 
When using a yoga ball do you ‘sit’ on it or ‘kneel’? If the latter why not give a kneeling stool a try?
It seems quite over-priced - you can get a cheap, slim electric one for one tenth of that! Hence my comment about it being a good business idea for someone if they feel they can make them cheaper (which seems very doable). Maybe even selling them as kits or a blueprint?
I highly recommend a standing desk - I usually feel like you do at the end of the day and at this point I move to the standing position.
I bought this one when it was in the sales and it is very sturdy:
Originally I used it with my old desk’s top, then when I found a wooden top I liked on sale I bought that. I love my desk now - it’s the perfect size and with no toxic materials! 
Try a kneeling stool Ben.. you can change positions on them by going from kneeling to sitting and various positions for each too. I’d never go back to a chair now. Here’s the one I’ve got:
