I've tried your build on my RB760iGS with a MT7621 SoC.
Compared to the official build of OpenWRT i got 200+Mb/s in IPv6.
That's mean :
160Mb/s in IPv6 with OpenWRT build, and 380Mb/s in IPv6 with your build.
But i can't get a 850-900Mb/s bandwidth... (excepted in IPv4 ofc).
Btw on RouterOS i can have 850-900Mb/s in IPv6.
It's a reason for me to search a another router with OpenWRT support and SFP port (still not found atm).
I'm sick of this, i've been looking for 2 weeks now (Solid-Run products, Banana Pi, x86 Mini PC, Embedded Routers, all "coffee Bin" products..).
Can't sleep correctly 