I am an experienced full-stack developer with Angular for the front end and .NET CORE for the back end. I have worked primarily with Microsoft SQL Server for databases. In other words, I have a programming background with various programming languages and frameworks. However, I am new to Python.
My question is: If I want to work with Python from here on out, what is the most in-demand for developing front and back ends with Python? Besides MS SQL Server, should I learn MySQL vs Postgres vs MongoDB? And what about the technology for the front end that is similar to Angular/React?
The entire point of the front-end/back-end split is that you can choose independently. Javascript (and variants etc. built upon it such as TypeScript) is more or less the only viable option on the front-end; there are implementations of other languages (including Python) in Javascript, but as far as I’m aware they are all very clunky. Once you have designed an API, it doesn’t matter to Python how the front-end produces a web page or arranges to make the AJAX requests - it only cares what the GET or POST request actually contains.
Similarly for databases. The choice of Python on the back-end has, IMO, little influence on the choice of database that’s suitable for your project (and some don’t really need a database at all). Most SQL flavours are fairly similar to each other; MongoDB is the odd-one-out - it’s classified as a document store rather than a traditional relational database using SQL for queries.
There are many back-end frameworks to choose from in Python. Many of them use overlapping common idioms; if you have to learn another one it will not be hard to pick up the basics. I would advise you to learn the fundamentals of Python first, making simple command-line programs; then choose any simple-looking framework and work through some tutorials.
Skills specific to web development, full-stack or otherwise, will probably not transfer well to data science or machine learning, regardless of what technology or programming language you use. They are fundamentally completely different problem domains.