Is it possible to use my existing HTML/CSS/JS with a move to Django?
HTML: Most of it will be reused, but you will do some work of splitting it into templates and blocks if you plan to generate it on the server side, which is normal for Django.
CSS: can be reused without change if you're careful with your HTML templates.
Javascript: it very much depends on the way you structure your application. Likely much of it will require rework where it interacts with server side (like calling APIs), but most would work as-is where it only interacts with the client (like form validation, etc).

Please note that the architecture you're describing and a typical Django architecture differ quite much; do your research and write a toy project first. There is a number of good intros. 
You will have to re-architect and rewrite all of your backend code. There seems to be Dynamo support if you stay with AWS, but everything around lambdas will likely go away, and you will deploy to normal AWS EC2 instances, or any other hosting provider if you want.
Remember: do the simplest thing that could possibly work. If in doubt, YAGNI.
I'm working on a full stack web application for a class, and originally was going to use AWS S3 for hosting, Dynamo as a database, Lambda for passing data to and from Dynamo, and Gateway API to connect the front and back end, but have had nothing but issues with AWS. As this is just a class project, nothing needs to be remotely hosted, and a classmate suggested Django to allow us to completely ditch AWS. After looking through a lot of the introductory stuff for Django, it looks like it could be the magic bullet we need. However, we already have dozens of hours sunk into our frontend HTML/CSS/Javascript, as well as some Bootstrap. My only reservation about making a complete dive for Django for handling our database and enabling us to run everything off localhost is that it seems to do EVERYTHING in Python, and I don't immediately see any way to integrate our existing frontend. Our frontend JS is intended to receive data from the database and do rudimentary calculations with it before providing it to the HTML page, and I'm also not sure how this would integrate with Django and SQLite.