Atmos sends full bandwidth to all speaker channels. So there is justification in using good speakers on all channels. If you highpass to a sub you only need to get down to the crossover point (normally 80hz).
it is just a matter of how much you care. Rear speaker quality does matter, I find I just don’t care if surround is accurate. I just want it not to break up and be pretty flat
Honestly I would looked for a used pair that more or less matches your fronts. Keep it cheap and replace that Polk sub. A good sub matters a lot more.