I have both the 10.5 and the 12 second generation.
I was using cases on both and the 12 was too cumbersome.
I recently removed the cases and use these naked as designed by Apple to be used.
The difference is amazing,
I use the 10.5 propped up by pillows in bed Twitter, Facebook reading news etc..
The 12 iPad I find I can research and write better with the split screen.
With full screen it’s a pleasure to type on the screen keyboard.
Split screen on twelve means you can have in effect two different computers.
If I were to choose I would choose the 12 iPad
But so that it is not cumbersome do not put a case over it, I store mine in a sleeve when not in use or if carried in a bag. eBay sells shoulder bags which should fit the 12 perfectly.
If you love books, surfing the net, lying in bed listening to podcasts and audio books the 10.5 iPad is the best, I fall asleep very quickly with an audio book, Rich Dawkins books recited by him and Lala Ward send me to sleep within twenty minute, and yes you can set the timer to switch off the audio after a time.