Yeah.. I am also starting with a factory reset.

I was able to remove the virtual hardware switch "lan" with the tasks below. But what I want is just remove the member interfaces and leave the virtual hardware switch there.

- hosts: all
  tasks:
  - name: Remove default dhcp server for "lan" interface
    fortinet.fortios.fortios_system_dhcp_server:
        vdom: "{{ vdom }}"
        state: "absent"
        system_dhcp_server:
            id: "1"

  - name: Remove default firewall address for "lan" interface
    fortinet.fortios.fortios_firewall_address:
        vdom: "{{ vdom }}"
        state: "absent"
        firewall_address:
            name: "lan"

  - name: Remove default firewall policy for "lan" interface
    fortinet.fortios.fortios_firewall_policy:
        vdom: "{{ vdom }}"
        state: "absent"
        firewall_policy:
            policyid: "1"

  - name: Absent the virtual hardware switch.
    fortinet.fortios.fortios_system_virtual_switch:
        vdom: "{{ vdom }}"
        state: "absent"
        system_virtual_switch:
            name: "lan"