I am getting ready to upgrade my current sim supported 5G openwrt modem/router and I want to get something that I won't have to upgrade for a while and get the most speed out of it. I am curious to see what you guys would do im my shoes or what you guys/girls would choose as the most capable device from the bunch.
Some x86-64 Sophos XG models have an empty mini-pcie slot where you may be able to install a 4g/5g model of your choosing.
"fastest" is hard. Generic system boards will not be able to support any fast 5G modem at maximum speed, primarily due to limited power available to the m.2 slots.
Mini-PCIe slots have the same issues, and cause additional complexity by requiring an m.2 adapter. You'll have to verify that both the slot and the adapter supports the required high speed bus and power delivery. Very few mini-PCIe slots have USB3 support, which means that you're limited to running modems in PCIe mode. Maybe not a big problem as you might want that anyway for "fastest". Using USB3 will probably max out at 3-4 Gbps. Raw guess. I have no system or network where this can be tested. But simply having a PCIe modem does not make it simpler to pull together a complete working system for OpenWrt. Expect a lot of research to figure out a working combination of system board, adapter and modem.
IMHO, it should be a lot easier to find a system designed specifically for a particular high speed 5G modem, and buy it with that modem already installed. Then you'll also get a working antenna configuration, which is an important to achieve this "fastest" requirement. In theory, I guess only mmWave 5G modems qualify for the "fastest" competition, making the antenna choices even more interesting. But there is little point in using money for such a modem unless the network supports it. In practice, modem and antenna choices should be based on the available bands, making "fastest" rather theoretical.
As for the OpenWrt support, this is pretty simple at the moment. x86 would be fine if there were any complete 5G systems. I don't think there are? Otherwise it's Mediatek, with something more powerful than the omnipresent MT7621. Now I don't know of any of such system either, but it's much more likely to exist. At least in the very near future.