Preflop
When someone has already entered the pot, you should generally have a better hand to get involved, than if you had raised first in from his position. T8s is a very marginal hand to open from EP, so when EP has already entered the pot, it should be folded from any seat other than BB.
Flop
You flopped an OESD with a BDFD, which is pretty much as good as it ever gets with a small suited 1-gapper. When EP miss his C-bet you HAVE to go for a bet here to thin the field, apply pressure and build the pot for the times, you get there later.
Turn
As played calling his delayed C-bet is a trivial decision, when he goes this small.
River
As played I would also give up. I agree with your own assessment, that a J or 9 is mostly going to call here, simply because the pot is still pretty small, so a regular bet is not putting a serious dent in his stack. You could maybe overbet, but in general I just dont like to play draws passively and then try to bluff the river, when I missed.
Conclusion
The key mistake in this hand was not betting the flop. If you are going to make this kind of casual call with a suited connector, then you need to be prepared to follow up on it postflop by using your position to make his life a hell, and especially when you flop a big draw. This is kind of the whole point, why we prefer a hand like T8s over Q4o.
If you bet the flop, bet the turn and jam the river, he is surely going to fold a hand like A9, unless he is a fish. And on a runout like this I would go all the way. The K was a decent
bluffing card, because it completed QT, and it apply pressure to JX and 9X. But you need to start telling your story on the flop not on the river.