so far as you have rendered

“We are very grateful to you for your kind help so far as you have rendered.”
Is this sentence OK? Please comment.
Can we make it a little more better?

No, it’s not right. You can say “We are very grateful to you for your kind help so far.” The words “so far” are optional, and suggest an ongoing situation in which more help is hoped for in the future.

(“We are very grateful to you for the kind help that you have so far rendered.” is correct English, but the inclusion of the “rendered” part seems a bit stiff and unnecessary.)

I am writing the rest of the letter below for your correction:
“We are sending our students for training in your institute.
We will be thankful to you for your great concern.”

We are very grateful to you for your kind help so far. We are sending our students to complete some training at your institution and would be grateful to you for your support.

(Though I wonder whether you intend this to be a request to send students, in which case, "We would like to send…’ should be used.)