The initial pull happens over more than .25 seconds. You'd be lowering the distance significantly. More than enough to walk out of or use a mobility skill our of easily, before Thresh can actually get to someone with his jump. You'd need to significantly increase the speed of the pull in order to have it cover the same distance. I don't see the 2.5 seconds of slow realistically being used against anyone who has any mobility skills, unless they were advancing into them. To give a comparison, think of Karma's tether. It has the same tether range as cast range, and it sucks for initiation even though she has a slow and a speed boost to help it. The reason is because it's easy to quickly break the tether even with basic movement. You need a significant positioning advantage on them to get it off unless it's being used defensively. Thresh's lowered pull wouldn't give him this kind of advantage.
The 1.5 second stun was a major, major, major strength of the skill. First, it serves as disruption in fights. Second, it actually is a major component of initiations, as it gives more time for your team to begin reacting, and for you to actually jump to the target (if you jump and he moves, you follow him). The stun is a major focus of why his initiation works. The jump into flay is significantly weaker for engages than the actual stun portion of the skill, because the stun is actually more effective at preventing movement and escape than the flay is.
To make this clear, I think you significantly underestimate Thresh's travel time when he jumps in. With the lowered displacement and stun time, it's going to be much easier for enemies to get out before Thresh can get in. It will also be much easier for them to get out even if Thresh jumps and flays, because they'll be further away from the follow-up and they'll have an extra 1.25 seconds to react. That's a lot of time to escape. If they have a mobility skill, that extra slow Thresh has won't be a factor.
As for less fun, the way I see it is that you're majorly gimping his initiation potential. You have absolutely not granted him a wider range of engagement. You've granted him a defensive slow. The most effective case will be using this to help someone kite away, not using it to engage. That's what tether abilities do. They work for zone control, but Thresh already has multiple abilities that work for that. This just cuts his initiation and hard disruption potential in exchange for something he already does more than well enough. That's fucking boring.
In order to make your idea work, you'd need to increase some combination of the initial displacement, the tether range, the actual effect, his jump speed, the strength of his flay, and the skill range. One thing I can tell you is that you have no justification for reducing the cast range if you're so thoroughly gimping the initiation potential. If anything, the range should be going up.
If you want to hit excess strength, hit damage. It's still high because it's barely been touched. His autoattack range could be reduced to make it much more risky to use his E passive. Let him be punished for missing a hook like Blitz is instead of still being able to trade well. Gimp those, not his core mechanics. It worked for Blitz, it can work for Thresh. If anything needs to be done with his hook, I could agree that the stun going to 1.25 seconds might be reasonable given the strength it gives in lane fights. Major reworks to cut his initiation potential aren't needed.
I read your changes, and I get the impression they're from someone who doesn't really care how the champion plays, just how they play against. That kind of balance is how we end up with Olafing and Skarner reworks. Breaking core mechanics and gameplay patterns is not nerfing intelligently. It's taking the easy way out. I'm sorry if that sounds kind of harsh or mean. I don't intend it to be, but I do think it's a game design trap that too many people take when trying to balance, and it's led to some of Riot's worst mistakes.
