I'm hoping that I may be able to get some advice. I'm looking for a new OpenWrt router that I use to router certain devices through VPN (ExpressVPN).
I think I will be using this router in addition to my main router (Asus RT-AC3200). (I tried running using Asuswrt-Merlin before but I had to reboot the router frequently). I may try and use it as my main as well, but only if it is stable.
I don't want to spend a lot but I would like a very reliable router, so I have been looking at models with a powerful cpu and as much ram as possible.
If you're 100% clear on not wanting to use its wireless capabilities, the wrt3200acm/ wrt32x probably provides more leeway for future WAN speed increases. IMHO the WRT1900AC/ WRT1900ACS is not cheap enough to compete against the WRT3200ACM/ WRT32x, the WRT1200AC is significantly slower.
In terms of raw CPU performance, WRT3200ACM/ WRT32x and r7800/ nbg6817 are very similar, with the ipq8065 based r7800/ nbg6817 being slightly ahead.
In regards to wireless support, ipq8065/ QCA9884 would be a clear winner, as mvebu/ mwlwifi has a couple of deficiencies that probably won't be fixed (no WPA3 support, interoperability issues with IoT/ smarthome stuff and more), however based on your description, this won't be an issue if you don't intend to use the wifi.
When it comes to wired routing performance, mvebu (~1 GBit/s line-speed capabilities) clearly wins over ipq8065 (around 400-450 MBit/s max).
In regards to VPN throughput, ipq8065 should be slightly ahead - but they're both very comparable.
At your current WAN speeds, both options (WRT3200ACM/ WRT32x vs r7800/ nbg6817) should work - although I don't think you'll achieve VPN throughput at full capacity of your WAN speed with either of them (this would need confirmation though).
A cheap/ low-power x86_64 board with two or more 1 GBit/s ethernet cards might however be a strong competitor, especially if WLAN isn't required.
I was considering a low power custom solution but I am looking for something compact. I don't intend to use the wifi right now, but that may change so I will look for a deal on the R7800.