If anyone is looking for a cheap multi-core MT7621A router that works with 19.07 - check https://www.amazon.com/gp/product/B01MXXQXW9/ - $90 and shipped from China
It's a ZBT WG3526 with 512MbRAM, 16Mb flash, should be 2 cores but shows 4 in /proc/cpuinfo. Comes with a custom OS that can be re-flashed to openwrt-19.07 using instructions for similar router: https://openwrt.org/toh/zbt/wg2626
I still need to properly test it, especially given [18.06.4] speed fix for BT HomeHub 5a, but it shows ~420Mbit/sec over close range 5Ghz wifi in default config (where wifi irqs are pinned to single cpu and RPS is enabled for all cpus but 0)