Instead of drilling down to the workspace, you can drill down to a user's directory (more on how to get this below) to list their schedules. A sample of the end result is below:
/api/v4/simple-scheduled-tasks?container=object:!tenant:defaultTenant~directory:__Root__~directory:__Users__~directory:183eb12b-f124-4ff1-9287-bdbd86be8db2
The user's directory (or rather, the whole locator which contains it) is within the same /api/v3/workspaces API call you're already performing. Instead of taking the locator field, take the object.directory field:
...
{
"object": {
"id": "991ad7cf-fa40-4b86-9164-e9473cddf01e",
"name": "gerard",
"directory": "object:!tenant:defaultTenant~directory:__Root__~directory:__Users__~directory:183eb12b-f124-4ff1-9287-bdbd86be8db2",
"description": "gerard"
},
"locator": "object:!tenant:defaultTenant~workspace:991ad7cf-fa40-4b86-9164-e9473cddf01e"
},...
Since you already have the token and the workspace APIs up and running, the only change would be to take the object.directory field from the workspace API result and use that as your container parameter for the /api/v4/simple-scheduled-tasks call.