Hello everyone.
If there is an M.2 Wifi 7 card that generally supports all hardware (Intel/AMD/Arm) and is 4x4, it is optimal.
No, at least not Arm.
For consumers there are three different options today, Intel as mentioned, Qualcomm and MediaTek.
Intel BE200/BE202
Qualcomm QCNCM865 (QCA FastConnect 7800/7900)
MediaTek MT7927/MT7925 (Filogic something or the other, also re-branded by AMD)
These are are 2x2 and support either 320 or 160 MHz wide channel width and the QAM (quadrature amplitude modulation) can also differ between the various SKUs, so check the specs before buying.
If not, which network cards support all hardware and are 2x2?
None of the above supports all hardware, simply because you need different drivers for Arm compared to x64 PCs. However, as long as you get the PCIe version of one of the above, it should work with Intel or AMD systems. Note that Intel also has CNVio2 cards, which only work with Intel hardware, so avoid those.
If there is no 4x4 Wifi 7 card - why not?
There are, but they're intended for routers and there are at least as far as I'm aware, no desktop drivers for them.
This is an example of such modules.
QCN6274/9274 WiFi 7 (802.11be) 4×4 MU-MIMO 6GHz Single Band Module
shop.compex.com.sg
Which M.2 is recommended?
M.2 2230 sized cards is the standard used on most computers today and it's the only ones you can swap yourself, as the M.2 1620 are solder down modules. The 4x4 router cards are usually not following the standard M.2 form factors.
Note that most WiFi cards use the M.2 A/E pin-out and thus won't fit in the M.2 slots for NVMe drives, as the A/E slots also support USB and some other interfaces that are used for the built in Bluetooth features.
Is there a link to a website with new/recommended cards?
I haven't even seen any comparisons between the different options on the market, so I'd say that's a no.
Is it still an advantage to buy a 4x4 router for a 2x2 card?
Yes, if you connect multiple devices to the router, as at least in theory, two 2x2 devices can then get their own 2x2 link to the router. If that always works in reality, is a different matter. Some of this is about compatibility between different brands of hardware.