The first build in the guide is always made to fit the meta and the runes present. The rest are situational and show meta builds that existed at some point in the years and that might cone back at some point in the future, if you are not sure about what to get or how ti mix and match then I suggest the first build that i list.

I think it depends on your own playstyle. If you are more the gank player you take Phase Rush, but if you like to farm more rather than ganking the lanes then you would go PTA.

I highly doupt it, i started LoL back then i was at junior highschool and i had nothing better to do and plenty of time, now my time starts to get more and more limited and and the little time i have left to play games i prefer to spend it on something more worthwhile honestly...

Try the builds on the top of the first page of the guide, those are the ones that get regularly updated, aka, first 1-2 builds.

He would be too op

That was around 7 or more patches ago, its a build with trinity and titanic hydra i think that i no longer use

The Movement Speed from Chilling Smite and BOTRK does insdeed stack as they are different unique effects, as long as 2 Unique effects don't have the same name then they stack, (i overlooed that Challening smite True Damage refreshes when you you attack but i still don't think it has enough value to be used), then i gank i try to get from behind of someone, using Chilling smite and BOTRK or whichever is ready, if none is ready then i just walk up to them, when i get close i keep atacking or if i have ulti i will knock them towards my teamates, i try to preserve my E as much as i can, the reason is that as long as my E is active it pretty much locks them from using Flash, as the E and Flash have pretty much the same radius so if they use Flash to run i just jump on them and they can Slowed and Killed, most Xin Zhao make the mistake of ganking a lane with E simply to burn they enemy's Flash, but why do that then you can simply kill them and maybe even amek them waste Flash at that. I sadly don't have a way to show the matches as i don't have the time to upload any (and currently i was in the Semester Exam period so i didn't play much, but during summer i will probably start playing again). For items generaly there is a marked patent in my mind, Guinsoo's for example as an item that works with stacks and has a high DPS its generally good when the enemy team lacks in CC and yours has at least a good number of them, generally it should be a situational item and not a core item as it gives a lot of damage only under specific circumstances, you can see my first 3/4 builds that i have listed in my guide, those are builds i have tested in hundreds or maybe thousands of games literally and they are not just there because i use them, each build along with the runes and masteries i give provides with a specific balanced set of Resistances/Health and Damage along with the proper effects and CDR that Xin Zhao needs, you will see that almost all of my builds include 30%+ CDR, generally i suggest your cores items be Bloodrazor-BORTK-Spirit Visage. Also i have tried Tiamat many times and its nice and all but i just don't think its better than other possible items that benefit Xin Zhao more, it has waveclear sure but Xin Zhao isn't really hurting on farming speed, usually i get more farm than the enemy jungler without it and with the help of my other items i get plenty of kills as well. Also about your Ulti usage, the ulti all in all has 3 uses, when i gank i always time my ulti by intuition after all those years playing Xin in order to properly use it on the enemy and knock them back if i need to,if i can't reach the proper spot fast enough and i want to be flashy then i start the ulti cast then i flash on the spot behind them and that what i pretty much cancel its animation that makes Xin staying in the same place, second use and most important is in teamfights, once you enter a teamfight or flash into it then you have to Ulti as many people as possible, that will give you a ****tone of resistances in order to tank the whole fight and pretty much kill everyone with the help of your team, lastly if you are dueling an enemy
(that will not run) with big duealing potential and you deem it enough to use your ulti then jump on him with E and use ulti right away as it does damage based on current health, making it more effective at the start of the duel. Basically as you said my builds provide with a lot of slows and Movementspeed buffs in order to catch fleeting enemies which otherwise would escape, i presonally think doesn't have the real need for more Duealing damage as to get Challenging Smite in order to kill the enemy team, all in all 90% of the time people will just try to run away from him and thus you need to be able to catch them. If you have any other question feel free to ask.