Minor nitpick. While patting her head after the fight, the piko piko sounds are Goltarou's ears twitching in her mute happiness as shown by the flowers too.
So what I got was that it is poki poki,
But that is the Romaji, not the TL.
The TL of it was pow pow or ping ping,
Which due to the hand placement I went with pat pat.
@Graviity Forgiveness. Like I said earlier its just a minor nitpick.
The head pat IS there (though not showing a nade nade/なでなで motion).
However it does clearly show piko piko/ピコピコ and shows her ears going up and down in 2 positions in that scene. Which is why I mentioned it. I do not have a raw to compare this to so I cannot really comment on a poki poki/ポキポキ sound.
@kuroro86 According to MAL this manga finished at 12 chapters back in 2019, it's long gone. The WN is currently on hiatus but the LN is ongoing and it's been licensed by Seven Seas, with all of the two currently released volumes properly translated. Just in case someone wants to read more of this.