In my case, I am not lookink for a newer one. I already have an underused atom which I can repurpose for the firewall. It´s a 2 core/4 threads 13W TPD processor. Its motherboard does not accept a lot o RAM though.
2core actually sounds doable.
I just looked at Netgate homepage and their slowest one is an 2-core ARM64 Cortex-A53 @ 1.2 GHz with 1GB RAM and seems to be able to do around 200Mbit throughput.
So with an x86 2core 4thread around 2.5-3GHz and 4GB RAM it should both be able to run half smooth around 250Mbit.
as soon i get some time over i will test where the magic breakpoint are