What I would probably do to accomplish this is to use a post function to set the assignee in your workflow using JIRA Misc Workflow Extensions. It's a paid add-on, but it adds so many useful functions to workflows that it's kinda become required in all my instances. Thankfully it's not super expensive.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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