The OP is a masterpiece. Months ago I replied here to a similar question by sharing my solution to the same problem. Let me re-post the relevant parts here:
I use a rooted Homeware (proprietary fork of Openwrt) router for gigabit WAN, it does its job for 20€ in total. Most drivers are proprietary and closed. You cannot rebuild or replace the kernel, nor the main squashfs. It's still some of Openwrt in the end, so everything you compile for userspace works, and some kernel modules work too. I run both Transmission (an USB3 SSD is attached for that) and strongswan roadwarrior server on top of it and it just works. Every proprietary driver is integrated into UCI so you always do things the openwrt way, including VLAN and multiple SSID setup. [...] There exist multiple models with different specs: mine (20€) can do SQM (at ~180Mbps) and has STA capable driver for the 11ac radio; others (20-30€ too) have newer firmwares, quantenna wireless, and hw crypto; I won't recommend the older ones with older SoC and wireless chipsets, but they may have matched your requirements as well. Almost all of them have VoIP ATA capabilities, sometimes you also get an FXO port or DECT radio. Depending on what you want from an Openwrt router, this could be a damn cheap and effective solution.
If you want a real Openwrt experience then x86-64 is the way, as someone told here. Leave Rpi4 to makers, it is not meant for networking and it is very expensive for what you get.
Meanwhile, since that old post of mine, I set this things up for my relatives and I succesfully got asterisk, F2FS driver and even hw-crypo running for IPsec. Recently, I started testing even newer models with integrated GPON (yes, up to 2,5Gb/s is possible), 11ax wireless and even ARMv8 64-bit dual-core processors at 1.5Ghz. The old prices I mentioned were referring to second-hand market, nowadays those old models halved in price. I don't know how much the most powerful CPU could get on SQM, but if you really need full SQM bandwidth then I think spending 300/400$ for professional single-function equipment is the way.