As a Xayah mid abuser, I could say this is a pretty accurate build but not really good overall.
In my opinion you should always goes Hail of Blade, bcs its best on trading, not many mid champ can win against the attack speed combine with the W and Q E if you time it right you can even kill them later or recall if you sense their jungle.
best spell should be ghost or teleport since it get you to lane faster and keep on the pressure.
for the item I think rushing Serylda to Tri-force is the best one, you can switch Serylda to Botrk since its good synergy with Hail of blade and can give you life steal plus anti tank.
This is my first response here and I my english is not that good so forgive me for my poor choice of words, hope you could consider this.

Hello. This guide could be really awesome, but in my opinion youshould explain more the gameplay (like laning phase, if roam or not, and so on). Alao, is First Strike viable for this playstyle of Xayah? I mean, you can poke a lot, and more gold=more dmg=more pressure. But it's just my opinion. Good work.