This still doesn't sound like a good reason to pay AWS instead of MongoDB. Didn't your Security, Cloud Engineering and Finance teams have no understanding of AWS at some point? Why can't they start to 'understand' how a managed MongoDB works, and give buy-in?
There's a common saying we have in our consulting circles, that "Processes should support business, but business decisions shouldn't be made because of lack of processes".
We had a large local bank choose not to pursue a good opportunity because "our procurement process takes too long". This instead of investing in fixing the procurement process.