Agents with 'assigned only' access lose sight of tickets they personally closed. How do I give them access to their closed tickets?

Switch from 'assigned only' to Department-based access. Add the agent to the relevant department(s). Department access is permanent regardless of ticket status or assignment -- the agent will always see all tickets in their department, open or closed. 'Assigned only' is specifically designed for agents who should only ever see their active workload, and closed tickets are by definition no longer assigned.