"Should I quit my banking job to become a C++ developer?"

I have a similar story in which I went after my passion for programming and quitted my job. Although my experience in other area is far shorter than yours. Being more precise, I am a finance graduate and worked in finance sector about two years before rotating my career sail to programming.
If you have enthusiasm for programming and feel that you have enough energy to work hard at least in the upcoming 3-4 years (in order to grow professionally in programming area), it's definitely worth to move forward. Being financially independent (that I didn't have when I made my decision) is another big plus for you. It's probable that you will make more money out of programming in a few years. Apart from all these financial considerations, being in love with work you do everyday is a great thing to have.

Go c++ if you want to get onto real time financial transactions but expect a steep learning curve even if you know c++ to a degree. Or choose java if you want to develop APIs for say open banking or integration. Either way, just be prepared to work within a governed framework where you don't have the same freedoms when you're working on your own projects. And good luck!

It does matter that you're not a junior, juniors at their age tend to catch information much faster because they have a fresh empty brain. In contrast, if you're older it may be challenging to remember things because while try to learn your brain thinks about something else ex: "your wife comes home soon" or "you need to accomplish a task"... You get the idea. Although if during all your years you maintained you brain capacity by reading books, learning constantly, and avoiding alcohol then it could be easy for you.

I am 31 years old, and have loved coding for the last 15 years. I finished my degree in chemical engineering 4 years ago and stareve programming professionally as a freelancer right after. Even though i had zero experience in programming professionally and had no experience dealing with other people's codebases or even working with other people in programming, i found work 3 days after i started looking. Since then, i have only had a week of not being on contract. My experience is that it is really a liberating way of working, but to do it you need the following skills, in order:
Communicative, self-motivation, Balls, development.
If you could see yourself freelancing, then If i was you, i would start by pivoting towards becoming a full stack developer by learning either Angular, Vue, React or Flutter. I know, i hated front end development too, but the joy of being able to create a complete end product all on my own is very liberating. Being self taught is not an issue, and your age might be an advantage. Since you know C++ i would advice learning golang+flutter for about a month 2 hours a day, before quitting your job, but those are just my two cents.

Java or other memory-safe languages (which would be most of them - C#, Javascript, Python, Go, Rust, etc.)? Sure. C++? I sure wouldn't. I know you say you can program in it, but that isn't what professional software development is about. It is mostly about working with other people's code, and that is a MUCH harder and less rewarding job in C++ than in any of the others, because it is a huge language, with many weird edge and corner cases that cause subtle and hard-to-find bugs, and that very few people know well. Pretty much all the people I know who actually can do C++ (and are therefore way smarter than I am, and I've done software development for 30+ years) are trying to get out of it, usually to do one of the other languages mentioned above instead.