"Assigned to" field in helpdesk has a domain to filter users and the domain use this computed field domain_user_ids to retrieve the users so you can override _compute_domain_user_ids compute method to apply your logic.

Or you can inherit the helpdesk form view and override the domain attribute of the "Assigned to" field.

If you have Odoo Custom Plan subscription, you can customize the form view using Odoo studio, otherwise you can develop your own custom addons to do that or using UI to inherit the form view.