I feel like you failed miserably with Xin and are trying to blame it on not yourself.
Because most of the stuff you're saying just doesn't have much in terms of factual basis.
Xin's damage doesn't suck. He actually has what are essentially 3 auto-attack steroids in AD, AS, and armor pen - which exponentially increase the potency of one-another, and also scale his AA damage to pretty absurd levels with AA itemization. It's on the high-scaling end, you just actually need some items for it to amount to much come late-game.
There are plenty of things you can do with Xin to make his knockup apply within a reasonable time-frame. Basic combo is charge in to slow someone giving you time to land the AA's, use Battle Cry for a %AS boost to AA faster, and proceed to land 3 AAs on someone. Several champions can land 3 AAs without use of a slow or %AS boost helping them out.
Xin's sustain is pretty solid for clearing jungle camps. He's one of a fairly small number of champions you can jungle without needing a full rune/mastery page at level 30. He's also a very strong early-game champion, the jungle is not an issue for him.
"Tanks" in LoL don't do no-damage. Mobility and Range are as good (if not better) at keeping a champion alive long enough to do their job, so there are tanky+DPS, mobilie+DPS, and ranged+DPS champions in the game.
Generally speaking Tanky DPS are called "Fighters" and "Tank" is a term reserved more for beefy supportive classes focused on innitiating fights, locking down priority targets, counter-innitating, and peeling. The level of tank doesn't differ though, it's the job you survive long enough to do with tankyness. And there are substantially more Tanky DPS fighters in the game than there are true tanks.
Xin will win a straight 1v1 vs. most champions in the game, tanks, fighters, or otherwise though. His mix of damage and CC make him among the best duelists in the game, if not the best for the most relevant parts of LoL for actually forcing a 1v1. He's not Tryndamere late, but everyone wants to 5v5 late.
Xin's base stats are above average.
If you think they're low, you're probably
- not cross-checking them with any other champion in the game
- confusing runes and masteries as being the same thing as base stats.
Xin doesn't lack CC. He has a slow, knockup, and AoE knockback. His CC is in a weird exceptionally good for a fighter spot where he can build full-tank and be a CC tank if he wants to.
His ultimate also has the potential for granting 125 armor and MR, which is the very definition of scaling well with tankyness, especially seeing as he certainly has the CC to back up dealing 0 damage and still being a massive threat.
For that matter - restoring 50 HP when you have 0 armor/mr will erase 50 base damage your opponents throw at you. Restoring 50 HP with 100 armor/mr (50% damage reduction) will erase 100 base damage thrown at you - so Xin's heal scales with resistances, and his resistances scale with HP.
The only thing you really got right is that Xin falls off late game.
That's entirely because he has to choose between carrying his assassin damage or tanky CC playstyles as there just isn't enough gold to pursue both directions in itemization, and itemization will often only aid 1 of those 2.
If you get a solid lead early-game you can amass the stats Xin needs late though, and he is an early game champion - so this isn't exactly a huge weakness.
