You're using these devices for NAT, I assume as all of your internal devices won't be internet-accessible at least on IPv4. So you don't just need routing, right?Thanks, finally someone has answered my original question.
Since I do not yet know how FTTH will be configured, I ask, why would such a Brocade or Microtik switch not be able to handle the routing?
And currently I have a Xiaomi Mi 3G (the original, V2 is horrible, and with a different chipset) running PandoraBox (will be running OpenWrt proper when I'll have the time), for routing (I also have a Netgear DM200 running OpenWrt and a newer modem firmware for vectoring, however that is for xDSL (VDSL2 here)), however I am currently trying to get either a Linksys Wrt32X or (preferably) the Wrt3200ACM, 2nd hand, so routing shouldn't be a problem.
Anyway, once you have a switch (media converter, or otherwise) that accepts the SFP, you can VLAN the WAN connection and handle the v4 NAT with a separate device, and it doesn't need an SFP port, just VLAN support. Or you could just set up access ports on the switch and connect 2 cables to the NAT/firewall to the WAN and LAN ports, respectively.