OpenWrt does support modern/ high-end filogix 8x0/ ipq807x based routers, it however generally doesn't support Broadcom based routers like that one (as wireless/ packet engine/ xDSL-/ cable modem and phone features are a no-go area (no driver) - and as a result very little attention is paid to their support). Given that OpenWrt can only support what some of its developers or other contributors (including random drive-by contributors) have sitting on their desk, paid for by their own money. Accordingly support does tend to trail market availability - and these contributors obviously prefer to spend their time of options with reasonable mainline support, a good chance for getting them fully supported.
With a throughput expectation of ~400 MBit/s, So you have 500Mbps-1Gbps fiber and need a router READ THIS FIRST is still largely valid. filogic 8x0 is a contender, but also x86_64, rockchip, RPi4+ (skipping the more exotic options like powerpc/ watchguard m300, the rb5009, various cavium/ octeon mips64 devices, etc.).