I would say a 2nd hand mini 4 is your best bet. There were overheating/processor problems with ios11 but they appear to have been solved by ios12 upgrade.
Mine has a new lease of life now. It's great to just slip into the arms and fly - so quick and easy. I have a mount I use too with a shade - so you have the best of both worlds
I have been flying my Mavic Pro, Mavic Air, and now my Mavic 2 Pro with an iPad mini 4. I love it.
I have an Android Shield tablet I use when I want live HD out. But it requires adapters and the battery life isn't as long as the iPad Mini.
A little update.
Bought yesterday an used ipad mini 4 and tested today in the controller clamp.
All was perfect, and i think the ipad mini is the best compromise between display size and device weight.
I've also ordered a tablet support (so i can try the mini with this setting, and also give a shot with the 9.7 ipad ) but for now i'm very happy with the purchase!