While all my Fire tablets do automatically update the Fire OS I have never had the update happen while I was using the tablet. The updates always seem to happen overnight.
I don’t quite understand what you mean by “pinned state”. Can you please explain. Thanks!
Yes, but after a reboot, when you pick up the tablet to use it, the Roon app is no longer pinned. I only use the Fire tablet as a Roon remote so I (attempt) to pin the Roon app such that it is always on the screen when the tablet is open. When you do that on the Fire tablet, half the time Roon is non-responsive and requires exiting out of the app and reopening it. I have been blaming the Fire tablet, since I assume that if the Roon app itself worked this poorly, there would be a lot more complaints.
If I don’t pin the app, I have to open the app again whenever I want to use Roon. That is a poor use case, since Roon forces me to use an app interface instead of on-screen.
The situation that you are describing happens with any and all tablets. What is important is how fast does the Roon (Remote) app reopen and once opened, how responsive is the app. I find that using the Roon app on the Fire tablets is about the same as using the Roon app on an iPad. Since the Fire tablets cost much less than iPads, I can use the money I saved on better quality bourbon. 
Get an iPad.
A basic one is fine.
Android tablets don’t work as well, as the app doesn’t optimally use the screen space and doesn’t respond to coming back up as well as an iPad.