imho, I don't think a53@1,2GHz can reach 400-500 Mbps routed speeds with OpenWrt when using PPPoE.....
Look at x86 mini computer/device for 1Gb, as suggested by @frollic
fwiw, similar query but at lower 500 Mbps
https://forum.openwrt.org/t/router-recommendation-for-3-bedroom-townhouse-is-netgear-r7800-a-good-pick/158487