Unfortunately you can't do this with groups and it's a shame because I know it's been requested for years. In my opinion Canvas messed up the feature request because they only let you set due dates by group with Group Assignments, and as you pointed out you need to be able to do this with with normal assignments where everyone turns in their own work.
Besides the manual method you are using right now this can be done by Sections; if you have access to create sections in your courses. Many schools take this access away from instructors so you may not be able to. But if you do, Canvas allows people to be enrolled in more than one section. So you can keep your original sections which is good since they might be tied to your Student information system, but then create additional sections like you did with groups. Once you create them from the People area you can click the 3 dots next to a students name, select Edit Sections and add them to an additional section based on how you want to divide them.
At my school instructors don't have access to do this but some of my admins will do it for instructors if they ask us nicely (just kidding, we do even if not asked nicely).
Hope that helps,
Rick
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