Why not to use default "Conversion completed" interruption?
The nice part is that in theory I can start an new analogRead directly after reading its value so the new value is (almost) ready when I need it
Usually I start analog measurements when I need a data at this specific point in time, not when I need a value that was taken "a while" ago. Start Conversion Timing for most application is critical, doesn't matter when data is ready, as soon as conversion completed before new measurements has to be done.