Option to anchor phase & milestone start and end dates relative to release date Merged

As a product manager, I move features through phases in a release. My phases are gather requirements, mock ups & JIRA cards, & development. Start and end dates of these phases can never change; example: development happens in sprints and they are always the 8 work days that end 2 days before the release date. There are milestones along the way that should be dependent on a phase or phases but can never move, such as Story Refinement meeting & Sprint Planning meeting. These milestones happen at the same time each sprint and are relative to the release date.

Currently, when I move a feature from one phase to the next, it adjusts the start and end date of the new phase. I need the feature to adjust to the phase start & end date or at least be independent of it so I can adjust it manually. Moving the phase around because you add a feature to it is risky and adds a serious possibility of getting us off schedule if it is not caught and adjusted.