2GB of memory is fine for openWRT. Routing is surprisingly light tbh, consider that most all home/SOHO routers run integrated SoC's with <256MB of memory.
routing speed is more dependent on CPU +cache speed
i'd eat my boot if a residential ISP let you run your own SFP fiber module. they have to pretty tighttly control those things to keep signal levels right and have wavelengths in the right spots. plus they'll need to upstream reconfigure it somewhat frequently as the local network changes and if it's not their hardware, they'll get mad.