Introduce or enhance planning process

Apply SAFe PI planning practice for a long term planning (3 months ahead)

Preserve principles of cadence and synchronization, i.e. support regular planning and cross-functional coordination, provide routine dependency management and support full system integration 

Upon Iteration / Sprint planning make sure that team's capacity is calculated properly, i.e. vacation and day offs are taken into account

Upon Iteration / Sprint planning make sure that load does NOT fully match capacity, i.e. there is some buffer preserved 

Upon Iteration / Sprint planning ensure that time is allocated for technical and quality debt activities, any kind of support and communication tasks

Upon Iteration / Sprint planning ensure that high priority improvement items worked out during Retrospective event are considered as well

For Iteration / Sprint planning organize space where the team can contribute into Iteration / Sprint goals and commit to them

For Iteration / Sprint planning control ensure that PO or proxy/backup is present to dictate priorities and supply team with additional information if needed

Put more effort into getting work ready prior to sprint, i.e. make sure that PO provides sufficient information, all features have technical solution and tech notes, work is split into smaller increments as shippable products, all artifacts (like designs, text) are provisioned on time, team understands what and how work should be done

Prior to sprint start, make sure that the entire teams makes final round of draft sprint backlog review to confirm that all questions are closed, and dependencies resolved.

Make sure to list all Sprint activities from development and testing standpoint ahead, like bug reporting/fixing/verification, code review and approval, communication/meetings, regression testing, refactoring and unit tests. All these tasks should be estimated and added to sprint plan if they are inheritable from Sprint backlog. I.e. committed scope will be composed from Sprint backlog including related / inheritable tasks and activities.

Introduce pre-planning session with team participants where the team can review product backlog with priorities set by PO and select items more suitable for the following sprint from DoR, absence of dependencies/blockers, compliance with tech solution and architecture perspective

During Sprint planning session take into account actual team's availability

Make sure that only "ready for development" work is taken into sprint

Confirm that team is aware of upcoming scope and confident in completing it in time

Set correct and precise Sprint goal

Allocate buffer time for other team's support, like another vendor, or client's team, or other stakeholders who can come up with urgent ad-hoc requests. Use time-boxed estimation during which the team can provide assistance/code review/consultancy to other parties, if actual time exceeds - then task/request should be planned to the next sprint. Then, evaluate time spent for such requests/support in Sprint to make this buffer more precise and targeted.

Introduce the rule that Sprint backlog should be formed with dependencies eliminated ahead

In case of a new team make sure to run several first sprints evaluating velocity and keeping it basis for further sprint planning

In the case of FF project - make a plan (schedule) to deliver contract values including all activities and workload with the focus factor. You could use Microsoft project or Smart Sheet for this purpose, or specific Jira addons.