My answer would be to create a similar but more senior position to said employees that pays a salary that is at least 5 times their current salary. Then tell them about it emphasising the out of hours responsibilities dangle it in front of them as a carrot ( this will yield its own benefits ) then if they are prepared to give more commitment for a greater salary then that will happen and if they don’t then you should be content with theses employees as they are.

You can then look outside the company for people to bring the extra commitment you need without feeling bad for bring in new people at a higher position and salary than your current employees.