@ryan29
"However, the limit of 200 channels per Team is quite low for what I was envisioning. I was hoping for 1 email request = 1 todo = 1 channel "
OMG, I tried a similar approach some months ago, i mean 1 email request - 1 planner task - 1 channel... found the 200 limit also 😥
THere is also a 400 tasks limit if you try to use Planner 'List tasks' to send weekly reports 😫
Anyway, was a great learning experience!
Now, you question is
"I want the adaptive card to be posted as a reply to the Teams message in this flow, so the topic in Teams would be threaded in a way that makes sense:"
Now I see what you are looking for, the idea makes a lot of sense. However, I am pessimistic, I mean when checking Teams connector I haven't found the way to post an Adaptive as a reply. According to the roadmap, Microsoft is putting a lot of effort to improve this connector in the following months, probably we will see new features and triggers soon.
Hope this helps