Using "just" is redundant when paired with the phrase "nothing more than", as they mean the same thing. You could use either of them, or replace it all with 'only' or 'simply', or anything else similar, but there is no purpose to combining any of them outside of wordplay. I've assumed you were using "just" for extra emphasis, so I swapped it for "really" as that works better here. Using "it" requires the context for what it is to be clear. With how you worded this, the only thing you could be referring to throughout is the project. This doesn't make sense for the sponsors to raise objections about when they already decided to back it though, so I'm guessing they raised objections about how you planned to complete the project instead, hence swapping the first "it" to "plan". However, this now means any future "it" would be talking about the plan instead of the project, so you also have to change the second "it" to clarify that you're talking about the project again.

I cut out a few "filler" words to make it more concise! The other suggestions are great, too. This would constitute a more simple version of what you wrote.