Out of "hundreds" I see 5-7 that look like decent home wireless routers at first glance (4+ gigabit ports, available new, b/g/n/ac, no unsupported hardware when running OpenWRT, <$150).
Based on that the original statement "I still don’t know any good reasonably priced routers which supports Openwrt." is very understandable. A lot of rows != an easy list of good routers.
This does not appear to be officially supported. On top of that, it appears to require one to use Xiaomi mobile software and accounts to set up the router initially prior to any flashing.
The "3G" model is not the model you linked. The 3G model requires you to flash proprietary firmware and then associate the router with a Xiaomi account using mobile apps for God only knows what convoluted reason. I would not trust this router even with OpenWRT running given the amount of blobs it requires and the hoops Xiaomi makes you jump through in installing an aftermarket OS.
I don't think that the Archer C7 hardware (or most consumer router hardware) supports any kind of encryption acceleration aside from what's built in to the WiFi radios. If you want a hardware accelerated VPN, you need to either get one of the Cavium-based Ubiquiti routers or something with a relatively high-end processor implementing AES instructions.
The lower routing performance has largely been solved in 18.06 with software flow offloading.
It depends upon your definitions of "good" and "router". If you are ok with <300 Mbps speeds, the Raspberry Pi 3B+ makes a surprisingly good router with openwrt. I suspect a lot of similar devices also work at Gbps speeds for very reasonable prices.
If you also need an access point, the cost does start to increase a little, but it still isn't bad at all, IMO.
I've converted some devices in the past, the latest one being a small TL-MR3020 box which now connects my Ethernet printer to my home AP.
Though I like openwrt, the web interface could be improved (meaning: making it more clear and simpler) because the above task, although trivial, required a lot more time than it should have.
Openwrt also doesn't forgive errors: on another little router I mistakenly deleted or disabled the wrong interface, and now that box is unreachable from anywhere until I'll take the time to open it and connect a serial port to reconfigure it from a terminal. But that's more of a feature :)
hard to parse this statement, since the software itself is doing the lion share of making the router "good" (unless you mean wireless g/n support or something?)
i've used this project to put new life into a Buffalo router that's probably 10 years old and runs way cooler and more performant than the original firmware. same with an ASUS router that's probably 6 or 7 years old.
Hardware is quite important; wireless standards keep evolving, some people care about how many ethernet ports they have, and system details (CPU, RAM, flash memory) determine performance and features (you lose a lot trying to fit your system in 32MB of flash).
Surely you mean you lose a lot trying to fit your system in 4MiB of flash, seeing as the stock images will flash to any 8MiB device with plenty of space left for configuration and the odd extra package or seven.
My grandmother has a SiteCom WLR-4000 (aka SiteCom WL-351, aka EnGenius ESR9850) and that has a 4MiB flash chip; building OpenWRT for it is a pain, but I can still get a flashable image that does everything I need it to do, including the stock web interface (LuCI), with 100k left for persistent configuration data.
It even has WireGuard (kmod-wireguard, luci-app-wireguard, luci-proto-wireguard) on it for a bridge to my router at my home.
When it breaks, or if I ever upgrade my WNDR3800, I'll give that to her. But for the time being it's still perfectly serviceable.
