- Is it possible to assign a user/team to a stage and only allow editing of fields for the assigned user/team?
- Is it also possible to prevent editing records that have been captured from a previous stage?
See illustration below

Note you can also leverage Column Level Security to secure fields by assigning your custom security profiles against those fields.
Further reading:
https://learn.microsoft.com/en-us/power-platform/admin/field-level-security