I have been thinking ----------- your proposal, and I have decided I would like to join you after all.
Fill the blank..

Question

I have been thinking  ----------- your proposal, and I have decided I would like to join you after all.
Fill the blank..

Answer

U may use both -  ' of '  and  ' about '
Secondly U don't need ( , )  since  U R using AND
Thirdly, since U have decided - good to say :  I have decided to join U
Alternatively :  AFTER ALL I have decided to join U,  AFTER ALL I would like to join U.

Answer

i think 2 options
1. think over
2.think about

Answer

Ans: I have been thinking about your proposal, and I have decided I would like to join you after all.

Answer

I agree with "about". Here's an alternate option: "over". "To think over (something)" means to give it serious consideration.

Answer

'about'

When you 'think about' something you have it on your mind and think about it actively in order to make inferences, decisions, or arrive at a solution.
In your example the person was processing the idea in his mind until he made the decision and arrived to the conclusion:  " I have decided I would like to join you after all".