[otnt]

In my opinion, the iPad Pro is the perfect size to read a classical sheet music and it is in 4:3. Unfortunately, the equivalent does not seem to exist on Android and MobileSheet is not compatible with iOS.

The largest ipad is 12.9".
In your first post you mentioned you already have Onyx Boox 13.3" which IS 4:3 ANDROID device larger compared to ipad.
IF you're looking at home at Boox 13.3" then that is the best device currently for reading the sheet music.
Not sure why you mentioned you want larger device?
There's new Samsung 14.6" Tab 8 Ultra (Android tablet not 4:3) coming early next year or there are portable monitors which can be connected to Android device if you want massive size for displaying multiple pages.
You can still buy used Pixel Slate or HP X2 Chromebook both with 12.3" screens (3:2 ratio), both running Android.