You've really just got cost, time, and scope to work with here.
Increasing duration will also increase cost due to related support overhead. Near a milestone I've done overtime with my teams, but I don't like doing that and never for more than a short spurt to reach a milestone.
If I were in your shoes I'd start recruiting now and factor in that vacancy until you expect you'll have someone in that role. Modeling that in your schedule will show either a slip in milestones or decreasing scope somewhere.