So I was able to do some benchmarks finally. We are moving soon to a new home, from the current 500/30 cable to 1000/300 FTTH pppoe internet. I bought the R4S hoping that it will be able to deal with this and maybe even do sqm on top.
I have seen it route gigabit with sqm but not how it deals with the additional pppoe overhead. I'm happy to report that it is fully capable of saturating Gigabit pppoe nat and even doing sqm over it with a+ rating on bufferbloat waveform test site. So I'm very happy with my choice, this is a perfect little router for Gigabit speeds.
speedtest page without sqm:
bufferbloat test with sqm set to 870:
all this over a pppoe connection.
I was using Anaelorlinski OpenWrt 22.03.5 (stable) minimal build. Schedulers were both on schedutil, irq on small cores, queues on the big cores.