Other than building out an extension that would automate the process I think the easiest way would be to have a CSV import for every day that you have a sub. You could build one for the morning that would give the accounts the subs are using access to the courses they need and one for the evening that would remove the access when the regular instructor is back. Yes someone would have to put together the CSV file to import and manage the file itself, but if you have a regular set of subs/courses needing subs or a large number of courses the upfront time investment could pay off very quickly.
Below is the Canvas documentation on the subject along with an example:
SIS Import Format Documentation - Canvas LMS REST API Documentation
AM:
course_id,user_id,role,section_id,status
123456,sub_teacher1,teacher,1B,active
234567,sub_teacher2,teacher,2A,active
345678,sub_teacher3,teacher,2A,active
PM:
course_id,user_id,role,section_id,status
123456,sub_teacher1,teacher,1B,deleted
234567,sub_teacher2,teacher,2A,deleted
345678,sub_teacher3,teacher,2A,deleted
If you know the courses and teachers (or the accounts they will be using to login) these assignments could be done very quickly from day to day with some planning. And that way you don't have a shared Admin profile at your account level.
Let me know if you have any questions.
-Michael