Not really an off-the shelf consumer router, but since you want to install custom firmware anyways, you might want to consider the PC-Engines APU2 board [1]. You can either install any "normal" desktop x86_64 Linux distribution or a specialized router OS such as OpenWrt [2]. The AMD APU on the board supports hardware virtualisation, so you're able to run several VMs via KVM to isolate the services the router is providing.
Of course this board doesn't come with the features of a fully-fledged consumer router, such as built-in DSL/DOCSIS modem, DECT, WiFi, etc, so your mileage may vary. It comes with 3 independent Ethernet ports and 3 mPCIe slots though.
I second this. I've been running PC engines stuff for a few years and it's great. I currently have an APU and it handles my gigabit fiber no problem. I use a separate off-the-shelf wireless router in bridge mode which let's me upgrade that independent of the PC engines (wireless hardware tech moves faster than router hardware tech).
I run openwrt on it and use the "adblock" package which works like pi-hole (minus the nice web stats). Having it be a plain x86 CPU is nice—For example, I compiled Telegraf on my local Linux machine (since openwrt doesn't have a package for it) and was able to just drop it on with minimal problems.
Of course this board doesn't come with the features of a fully-fledged consumer router, such as built-in DSL/DOCSIS modem, DECT, WiFi, etc, so your mileage may vary. It comes with 3 independent Ethernet ports and 3 mPCIe slots though.
[1] http://pcengines.ch/apu2.htm [2] https://openwrt.org/toh/pcengines/apu2