"I truly appreciate" is correct and usable in written English. You can use it whenever you want to emphasize how thankful you are. For example: "I truly appreciate all the help you have given me".
(i truly appreciate your feedback).
I truly appreciate it.
I truly appreciate that.
And I truly appreciate that.
I truly appreciated your fine article on James Stockdale Jonathan Mahlerr).
"I truly appreciated that," Manning said of the tribute, which was the idea of the Colts' owner, Jim Irsay.
Delicious winter apples from Stanthorpe almost changed my mind, but it wasn't until I moved to England that I truly appreciated their brilliance.
Awesome tool! I started using it one year ago and I never had to look for another app
Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia