I'm not sure how a 'waterfall'-style development lifecycle will help you here. Even if you front load all of your design, documentation and implementation, you will still find bugs and security flaws in your software. As an example, see every software product ever shipped!

I think you're maybe labouring under a misapprehension of what agile development means. In particular, you don't have to 'constantly update' deployed software in order to be able to develop in an agile fashion. You're free to follow the guiding principles to make sure you are able to cope with changes when they inevitably happen.