Early game its completely fine but late game when literally every single second counts that 2.5 second reload speed could cost him and his team a fight.
Make it so his Reload is lowered with Ultimate ranks down to like 1.5-1.8 seconds at level 18.
Then he would be broken. That reload time is the only thing that prevents him from continuously spamming one shots.
Late game he should have 100 percent crit, and sky high damage values.
Iscreasing reload at this point in the game would lead to nerfs.
Further more, you still have plenty to do. You have to avoid getting dove, you can be laying traps, using W to snare enemies. A good jhin will not be hampered by this.
His reload is probably about .5 - 1 second longer than it needs to be. However, if they just changed the reload right now his damage would have to be nerfed to compensate. Especially late game he'd be doing just stupid amounts of damage from the extra shots gained with a faster reload.
His biggest problem is immobility alongside long time windows to punish him. Adcs without some sort of dash / on demand quick movement ability are just horrible in the current meta. He can't exactly get out of the line of fire and is essentially a sitting duck throughout a good portion of the fight when he's not just acting a ult / stun bot.
Jhin's kit would be better for a immobile mid laner than a adc. We're literally trying to shove a square peg into a triangular hole by taking him and forcing him to be balanced as a adc.
His winrate has gone up to 50% so the latest buffs were successful at balancing him. It's just his kit feels ill fit for the meta and role that he's being played.
Think of it like other abilities (aka: spells, since his aa is unique and doesn't follow normal aa rules).
They have cooldowns for a reason.
All they gotta do is reduce Jhin's reload time by 1% (multiplicative) for every 16 bonus ms he has after the 4th shot up to a cap of 20%
Would open up more build diversity and fun without breaking him.
Any lower than what I suggested is just insane.
