For things like multiple DHCP/PD servers for multiple networks, subnetting (with dynamic prefixes), firewall rules, etc.

You can actually ditch the GUI and go to the command line with OpenWrt and do custom/weird/power-user stuff to your heart's content.

Well, yes. But I can do that with a nice UI in Mikrotik.

Other things: switch port management and bridging, proxy ARP, and so on.

	▲	cassianoleal a day ago \| parent [-]
		What about switch port management? I can't remember the last time I couldn't do something with switching on it. Not sure what you mean about bridging either. I've never had a use for proxy ARP so wouldn't know, but I do know it's possible to enable it. If not on the GUI, at least on the CLI.