My preference would be to:

  • Disable all services on the AR300M; all services from the AR750
  • Select one port on the AR750 to be the trunk to the AR300M; run everything as tagged, untagged packets to the switch’s bit bucket (VLAN 4095 or an otherwise unused VLAN)
  • Set up the “WAN” physical port on the AR300M as your trunk; run everything as tagged, untagged packets to the switch’s bit bucket (VLAN 4095 or an otherwise unused VLAN)
  • Set up the other physical ports as desired for wired connectivity to various VLANs, tagged or untagged as your clients (including any additional switches) require
  • Bridge the VLANs within the AR300M as desired across the wireless interfaces
  • Add firewall rules to prevent cross-VLAN forwarding and, if possible for your configuration, disable forwarding at the kernel level

802.11r would be an interesting later experiment, but my experience is that few clients other than iOS support it.