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Yes, I was just about to write about that at the very beginning – what is the purpose of the checkbox that no one uses? In general, even it doesn't completely solve the problem, because in the system I proposed, the timer doesn't accumulate, but with the checkbox disabled and added time, it does accumulate, which will still force players to start their turn as early as possible.
I wanted to express my idea from the very beginning as follows – to remove the unnecessary checkbox and replace it with the interface I proposed.
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Perhaps the following argument will be more convincing:
You can record a video of your game on your computer, and in the intervals between turns, watch a video of the previous turn and think about the next one, thereby saving time. (This also applies to viewing air and earth spells, the results of which, for some reason, cannot be viewed during the opponent's turn).
The same result is achieved with the interface I proposed, only now the time for thinking about a turn will be limited.