Being assigned on a task wouldn’t give those users access to edit your approval flow automatically. Approval processes themselves (which are used in any task that uses that flow, so more than just this specific task) can only be edited by admins, I believe. If you have ever-changing flows, you may want to consider single use approval processes, since if you change something in a templates approval flow it may have impact on other tasks using this flow. Or you can consider having another approval flow with the different users you need so they can choose the appropriate flow. If it’ll always be contingent on who the task is assigned to, then I suggest using single-use (ad hoc) task approvals, not a templates flow/process.
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