128 MB RAM for a dual-ath10k{,-ct} device are insufficient. IPQ40xx is a solid SOC with decent wireless capabilities (and I'd rate it higher than mt7621a+mt7603e+mt7615e), but the RT-AC58U should not be considered as an option, basically all other devcies using this SOC come with twice its RAM (and they need it).
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Yes, kmod-ath10k-smallbuffers mitigates this a bit, still, ipq40xx devices with less than 256 MB RAM should not be considered or bought.