Please keep in mind that kernel support for RK3588/RK3588s right now is very special. All RK356x/RK3588(s) thingies are somewhat based on Rockchip’s reference designs and they both tested and optimised driver support for a limited set of Wi-Fi/BT chips only.
So when searching for M.2 modules that will work flawlessly most probably it’s a good idea to look around what will be used on devices having to use Rockchip’s recent BSP kernel (as such we can look at RK3568 too) and what is already confirmed to work. Also keep in mind that if certain interrupts are not routed properly stuff like powermanagement (waking up BT from sleep and so on) doesn’t work.
- Firefly M3, Rock 5A: USB2 internally attached RTL8852BU-based Fn-Link 6252B-UUB providing 802.11a/b/g/n/ac/ax and BT5.2/BLE5.2
- M.2 key E card in Rock 5B review sample: PCIe attached Realtek RTL8852BE-based Fn-Link 6252M-PUB providing WiFi 6 and Bluetooth 5.2
- Khadas Edge2, Mekotronics R58, Mixtile Edge 2, RD-RK3588 development board, UGOOS UT8 PRO and Orange Pi 5: PCIe attached dual-band 802.11 ax/ac/a/b/g/n WiFi 6 2T2R and Bluetooth 5.0 via Ampak AP6275P
I would buy this thing here to be better safe than sorry: https://wiki.radxa.com/Rock5/accessories/radxa-wireless-module-a8 (which according to @Jack is a ‘WiFi 6/BT5.2 M.2 card’ as such I would guess it’s also based on RTL8852BE-based Fn-Link 6252M-PUB?)
Update: (not too) surprisingly a M.2 2230 key E card with RTL8852BE is also recommended by Ameridroid (see at the bottom here).