What is the difference in meaning between "be going to be doing something" and "...

Is there any difference in meaning between be going to be doing and will be doing? For example:

I can't go to the party as I am going to be working on my project.

I can't go to the party as I will be working on my project.

Does the first sentence imply that the speaker had more agency in their decision making?

I would agree that the first sentence hints at a touch more agency.

The most idiomatic way to say this, in my opinion, would be:

I can't go to the party because I'm working on my project.

"Going to be" here is implied but not stated; enough context is supplied to make this unambiguous.