Sensei Project Solutions has a set of quality standards for Gantt charts that is the best source for project scheduling I've found. (no affiliation). Two of their rules are:
with the exception of project start, every task on the Gantt should depend on another task.
with the exception of project end, every task should be the predecessor of another task.
Your landmarks don't follow this rule - I'd omit them from the chart. The landmarks are hard scheduled, coupled with the calendar; their schedule is decoupled from /not dependent on any deliverable.
If you include them on your Gantt chart, they will play havoc with your critical path, your slack calculations, etc. They will interfere with your ability to manage your schedule.
If you must include them on the Gantt, then I would create a separate deliverable (numbered higher than all others) that includes all the time fixed tasks. That will permit you to account for the time & resource constraints. (Call it oversight).
In my opinion, Gantt is useful for schedule management; people greatly overestimate the utility for resource management. Most of us don't work in environments where Microsoft Project's assumptions about resource management are useful.