i had this issue too on my omnia device and v4.0.3.
i replaced:
ath10k-firmware-qca988x - 2018-05-12-952afa49-1.12
kmod-ath10k - 4.14.158+2017-11-01-10-0a66bb0316b4402bf65555c64ceed313.2
with
ath10k-firmware-qca988x-ct-htt - 2018-05-12-952afa49-1.20
kmod-ath10k-ct - 4.14.158+2018-03-16-30827f7d-3-0a66bb0316b4402bf65555c64ceed313.2
and for me it is more stable, no firmware crashes, but i did not do a longtime test.
i also tried the latest official firmware from december 2019, but it was also unstable. the ct-firmware (htt-version) seems to run quite fine for me.