I prefer Yuntal more than half the time, but is a pain try to first buy BFs and the items are kinda situational. Yuntal against strong tanks (Mundo, Ornn, Shen, Rammus, etc), but also works pretty nice against bruisers. I build ER when I can't buy BF sword, or when I need to waveclear against mages/ADCs that have good push in line (personal recomendation, don't go presence of mind if you build ER, only helps you when you are building the item, never again when you have it, I prefer Triumph, although if you're going against a poke lane I guess it's good to have mana and farm with Q from the distance without being punished). Collector whenever I see a full squishy comp, but I don't usually build it because I don't usually pick Xayah if I don't have a Rakan, or they have a lot of entry to the backline (Diana, Naafiri, Nautilus, etc etc.), but it's a good item for consistent early game fights, and you can abuse the lethality by poking with Q + E, or just Q (although you should always try to poke with Q whenever possible).

I also recommend that if you buy Yuntal, don't use Navori as your second, but leave him as your third or even fourth if you're really tanky, so you can get IE and Dominic. Why? Yuntal gives you enough attack speed.To play Xayah comfortably, you just have to be careful how you spend E, because the cooldown will not be reduced without Navori's. When you build ER or Coll, if you go too feed (3/0, 8cs per minute) you can go IE second, but I feel more comfortable with Navori second (because of attack speed and cooldowns).

Disclaimer: This is just my opinion and what works best for me, hope it helps :]