If I am not mistaken there are already some banks using Mendix for these purposes. It now even might be easier with the native mobile options. So in my opinion I would say go for it. Create a small test project so you can get a feel on how Mendix works. I do not know if your current dev team already has Mendix experience. Mendix has people to help you get your first dev team up to speed. Contact Mendix if you want to go that route.
Mendix is capable of building any application that is required by business. Banking software, ERP, CRM, Hybrid mobile app, Native mobile app….
The only catch is, while creating bank software, your data will be PII and PCI. This means that your application that will deal with this data will also come under PCI and PII scope. This means you will need to deploy these apps on PCI/PII server space. So you will have to use your own server to deploy the app, which is not a issue as Mendix supports it.
In short, you can use Mendix for the banking app that you want to build.
I agree with Romil and Ronald. And what you are describing does not sound like you need to add a lot of customizations. Or do you have other use cases that really need customizations?
Thank you very much, my mind is now at ease about the use of Mendix in our projects. A community such as this is sure to make Mendix grow further.