How about kindest regards or kindest thanks?
Superlatives are
in formal communication (imo). Excluding rare circumstances we cannot know the highest degree of something--even our own feelings.
The "regards" in "kindest regards" already implies fondness for the recipient; the "kindest" is thus redundant. (I feel the same way about "best regards" but virtually no one will raise an eyebrow if you use that phrase. Descriptive grammar, it's so demonic friends.)
I know we can sarcastically or backhandedly thank others, but generally speaking one does not offer thanks in that manner, so describing them as being kind is unnecessary and awkward. Do you thank others in a mean-spirited manner on the reg?
(Thanking someone kindly is redundant too, but the question posed by the OP was whether or not it was presumptuous, not redundant.)
All this nonsense probably occurs because we shortened "Would you be so kind as to [action]?" to "Would you kindly [action]?"
