I'd say that there still is the mystery/suspense factor if you play TJ first. In this case, the question isn't about whether it was Chase who killed Sydney, but it becomes a question of who wrote the letter and treasure hunt. After playing the other routes, we know that multiple monsters exist within Echo and so every time something supernatural happens we, the reader, just blame the monsters. But since they aren't explored in TJ's route, my suspicion bounced around between Carl, Flynn, TJ, and even Chase himself. So it was very surprising to learn that I had failed to consider the possibility of Sydney (as the socket monster) or another monster being the author of the letter. And it made me immediately want to go back and try other choices and routes to see if I can learn more about the monsters.
It's never made clear who wrote it, but it definitely wasn't Flynn since Flynn didn't recognize the real final letter when Chase showed it at the lake. Chase only made the fake letter. Therefore I speculate that it's likely created by one of the mysterious Echo entities, which would explain why the letter could look old and is written in Sydney's style but touched on evens after his death.
Yeah that was my theory too, and the way the drawing in the final letter seems to have been drawn in TJ's style (like the one he drew at the diner). However one major problem with the theory is that if it was TJ who drew it, it wouldn't have made sense for him to go berserk on Carl at the end. Also it wouldn't have made sense that TJ did it in terms of the timeline. Chase was pretty much always with TJ all throughout the route so he wouldn't have had the time to sneak away and set it all up.
At most, TJ could have drawn the final drawing, and then one of the entities took it and put it in the last area as a way to trigger the hysteria. The evidence for this comes from Flynn's route, when the gang found out that Chase was the killer and the hysteria began immediately (which we're assuming is what the entity wants). So when Chase swapped out the drawings, the hysteria was narrowly avoided and the entity was forced to directly manipulate Chase (through the mirror). As a result the hysteria was delayed to 2017 based on the epilogue.
I don't think TJ attacking Carl contradicts anything, it's clear he's heavily repressing the memories of Sydney's death so even if he was the one behind the treasure hunt he probably wouldn't have been all there mentally while setting it up, he might have even been influenced by the town.
It's true he was often with Chase during the day but he still had plenty of opportunities to do it, especially at night. It's also possible he might not even have set it up during their trip to Echo but instead while he was still a kid, which would explain why the letters look old if it's not done purposefully. Another thing that points toward TJ is that he had a pretty clear chance to put the letter that begins the treasure hunt in Carl's crawlspace during the birthday party since he went searching downstairs.
Concerning the hysteria and the entity I don't think we have enough information about them to make an accurate guess on how they operate, if anything the entity controlling Chase was pushing him towards concealing the truth not revealing it.
Individually, the facts point to TJ being the one who wrote the letters, but taken together they begin to clash with each other. When TJ attacked Carl, it ruled out the possibility that TJ was aware that he was the one who wrote the letters (since he wouldn't have directed his anger specifically at Carl in that case because he would have known the letter was fake).
However, this then throws the other parts of the theory into question. If TJ did wrote it all unconsciously then he would have had to write the letters in Sydney's style, made the letters look old, go around town to place the letters, all the while unconscious. It was also strongly hinted that the letter in the school building was unnatural, and that it would have been looted years ago if it was really that old (which puts doubt onto the theory that TJ made the letter before the trip).
Then there's the matter of the clues themselves. The clues are passively hostile towards Chase, suggesting active dislike or hate towards Chase (which only really Sydney and perhaps Flynn feels that way, more on that later). Given what we know, I find it hard to believe that TJ would come to hate Chase subconsciously. TJ has always hated Sydney so there would be almost no reason to hate Chase after Chase saved him from drowning. And if it was subconscious, we would have seen some hint of the hate bubbling up into his consciousness every now and again. TJ may have held the pain and guilt, but hate just doesn't make sense for him.
"if anything the entity controlling Chase was pushing him towards concealing the truth not revealing it"
Did it though? We don't know for sure if TJ would have told Flynn the truth, all we have to go on is the entity's words (and it lied about Flynn knowing Chase's secret, and also about breaking the cycle by killing Flynn). And the reason it helped Chase was probably because it didn't care about revealing the truth, but in getting hysteria events to happen. Taking the other routes into account, TJ's route is very unique because of 2 things: 1) it's the only route in which the hysteria didn't happen in 2015, and 2) it's the only route in which the entity speaks directly to Chase.
With the hysteria, I think it's because this is the only route in which the group is completely destroyed, splintered apart. So if the truth were to be revealed, it may not have been enough to cause the required emotional reaction for the hysteria that the entity wanted - so the entity decided to cut its losses and prepare for the next hysteria (by creating a new secret for Chase to hide). Evidence for this can be found with Flynn. As he was dragged under water he realized the truth, yet the hysteria didn't happen because his emotions alone wasn't enough (based on information from Murdoch's route in TSR, the entity feeds on emotions).
As for the other unique thing with the route, the entity during its talk with Chase admits that the town is almost saved, that it may actually survive. Or in other words, its plan was almost foiled. From that information, it is my current theory that it was the socket monster who wrote the letters.
Remember how the clues contained a dislike for Chase that only Sydney had? What if it was actually Sydney who wrote them as the socket monster. From Flynn's route we know that the socket monster is an amalgamation of multiple people/entities. And just before Flynn was absorbed himself he got to meet Sydney, suggesting that like Flynn, Sydney was absorbed into the monster. And we know from Jenna that the monster has always been there for her and protected her, which would give it a motive to foil the other entity's plans. Looking at the result of the letters, the letter was at least successful in getting Jenna checked out of Echo early and out of harm's way.
And if it was the socket monster, a couple of other random details just falls perfectly into place. Being a supernatural entity, the monster could have easily forged the letters on the spot and made them look old and placed them anywhere anytime. Being another entity like the one that spoke to Chase would have also given it the power to oppose another powerful and manipulative entity.
That said, the theory does have a couple of small holes that I've yet to work out. The biggest one would be how close the letters drove the town to hysteria. If the socket monster's aim was to protect Jenna and perhaps stop the hysteria from happening, it came very close to triggering the hysteria through the final letter. If Chase hadn't swapped out the final letter, there was a real chance that the reveal would have caused a hysteria similar to the one in Flynn's route. So does that mean the monster knew that Chase would swap the letter? Or did it manipulate Chase into swapping the letter? The other issue is that if Sydney is conscious as the monster, why does it protect Jenna? Why not someone else like Flynn?
Of course, my theories regarding the entities are far from conclusive. We'll likely have to wait for Arches to get some concrete information on them :/.