Simply: "Easy" way handle 1 Gbps router is Rasp 4 with extra (USB?) network card. Maybe I understand right is it a good way for this.
But: How about "i86"? My present routers are i7-3770 3,4 GHz. So, normal tower computer. First network is any gigabyte fast card, second any. If I take two fast network card, maybe this is ok. Or, how it is possible calculate and check what is enough? "Evidently in an ideal world, we should have maybe 1.2GHz processors or better,..." found from this text. So, is this type router enough for 1 Gps network? Or is it any way calculate what "power" is enough?
That x86 CPU should be able to route firewall traffic shape and more at 1 Gbps. Compared to a raspberry pi4b it might sip a bit more electricity during high load phases, but performance wise this should be okay, if configured correctly.
However Ivybridge generation is a bit power-hungry... I operate an Ivybridge Xeon with similar specs in an HP proliant microserver which draws ~45 Watts when idle and >= 90 Watts when pushed. After measuring these numbers I stopped running this 24/7 and power it off hard via a switchable power strip, because even in stand-by this thing draws 5 Watts (which is close to what a raspberry pi 4B will draw as router).
Yes, your existing setup will be fine. If you wanted to make the leap to an ARM-based device (for power efficiency, space savings or whatever else) I would suggest the Raspberry Pi 4 if you can find one or a NanoPi R4S.
An ivy-bridge i7-3770 is way more than needed to do 1 GBit/s wirespeed, even including SQM - an ivy-bridge c1037u could do that in my tests; without SQM, baytrail-d Atom or newer can do the job (easily) - modern Atom probably including with SQM. More modern x86_64 systems will save you a lot on electricity - and a carefully selected system might pay for itself within rather short time.