Realistically, you don't need security, NAT alone is enough since the packets have nowhere to go without port forwarding.
But IF you really want to build front end security here is my plan.
ISP bridge -> WAN port of openwrt capable router with DSA supported switch (that is almost all of them)
Set all ports of the switch to VLAN mirroring mode
bridge WAN and LAN sides
Fail2Ban IP block list in the bridge
LAN PORT 1 toward -> OpenWRT running inside Proxmox LXC (NAT lives here) -> top of rack switch
LAN PORT 2 toward -> Snort IDS
LAN PORT 3 toward -> combined honeypot and traffic analyzer
Port 2&3 detect malicious internet hosts and add them to the block list
(and then multiple other openwrt LXCs running many many VPN ports as alternative gateways, I switch LAN host's internet address by changing their default gateway)
I run no internal VLAN, all one LAN because convenience is more important than security in my case.