However this means that Activities estimated to take only 1 or 2 hours cannot be used in planning as such - for planning, they will always take at least about a day, right?
No, duration will be displayed as 1 day, but planned work will take just 1 or 2 hours.
So if I have a bunch of smaller issues / todos, I could combine them into a single, larger Activity and document them there.
However, I then would have to manually keep track about the resolved and still open TODOs within this Activity. An example would be an Activity which has 10 sub-TODOs.
Maybe I could use Tickets to model this? Those could then also be linked to an Activity for time/effort tracking, if I understood the manual correctly?
This is exactly why we have "Tickets".
You plan an activity to treat tickets.
You enter detail TODO on a dedicated ticket.
You link the ticket to the activity (with planning activity flied)
You enter work directly on ticket, it is automatically subtracted from left work on planning activity.
How is it handled in this case if the tickets are assigned to and resolved by different resources - will the Activity stay unassigned, or will each Ticket have to be linked to an Activity assigned to the same responsible Resource as the Ticket?
You just have one activity. All ressources are assigned to the acitivity. Each resource enter some real work on the ticket... That's easy (see button "Dispatch work" for more detailed input mode)