Learn English – thanks for help or thanks for the help or thanks for your help

I think that they convey roughly the same meaning.

I think that in the case of

you were sweet to help me

The to doesn't indicate a purpose in the same way as

I save money to buy a house

It can be thought of more like

It was sweet that you helped me

Though I guess in an abstract sense, it does mean "Your purpose (to help me) was sweet".

In case you or someone else is curious, here's how you might respond to someone using the phrase in a more general sense

You are handed a very nice gift

Aw, thank you. That was very sweet of you

I can't think of a commonly used sentence that would convey this meaning, but there's nothing wrong with writing, "Thank you for being available" or "Thank you for making yourself available."

We do sometimes thank people in advance and sometimes we note that the thanks is given in advance: "Thanks in advance for your help."