Small speakers plus sub can actually be better than floor standers if the amp.offers proper bass management.
The key is that the optimal placement for best low bass response is not ecessarily identical to the best placement for the main speakers. In particular, you can place the sub close to a wall or even to a corner and use EQ or room correction to tame any bass boom that might occur with such placement. This saves amplifier power for the sub and can provide a more equal pressurisation if the room.
WiiM's built-in digital crossover will relieve the small Diamond 12.2 from what they cannot do well, heavy bass duties. So, less thermal stress and less cone movement for their tiny woofers. This lowers the distortion, resulting in better sound.
If you ask me, go for a sub, not bigger speakers.