Since the WiiM Amp will do all the low pass filtering of the subwoofer and high pass filtering of the main speakers, the subwoofer of your choice should ideally have an LFE input, bypassing the sub's own crossover. Some subwoofers don't allow to set the crossover point high enough to not interfere with the crossover set by the WiiM Amp.
You don't need continuous phase control on the subwoofer, since the WiiM Amp allows to fine tune the delay between subwoofer and mains very precisely in 1 ms steps. You don't even need a phase switch (0° or 180°), since the WiiM has that too (but there are probably very few subwoofers that come without it).
There's no need for any fancy DSP processing on the sub, because - once again - the WiiM Amp can do the same thing. Of course, if you end up wanting a subwoofer with its own DSP and/or room correction, that's no reason to avoid it. You could always find out for yourself if you prefer the sub or the WiiM Amp to do this for you. But you can probably save some money by buying a more basic subwoofer.
If the subwoofer you are looking for offers high level inputs (to be connected in parallel to the loudspeaker output of the amp): Simply don't use them.