For this solution, you don't need a plugin - but it's not perfect as you'll not be able to edit the fields when editing the issue, only in transitions between statuses.
-> Use a Screen with these fields on it for transitions in the first half of the workflow only.
-> Use a Screen without these fields as the standard edit screen, and in the second half of the workflow.
Or in more detail
a) Make sure that the fields are not on the Edit Issue or Create screens for this project.
b) Create a screen with the fields on it.
c) Open the Workflow in the editor. Open each transition before "Sizing Complete", and set it to use the the screen from step b (with all the fields on it). Your users can now edit these fields when they use these transitions.
d) Click all the transitions in your workflow after Sizing Complete, and make them use the screen without the fields on it. Your users now cannot change the fields, since they are not shown when Editing the issue, and they are not shown in the transitions after "Sizing Complete". To edit the field, they would have to send it back to a status where the transition screen contains your fields.)
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