Unix already have a security model for this: User and Groups. You could create a new user called `banking`. And used that exclusively for any banking activities. Especially with the X11 model where you just have to type `startx` and not have to launch a bazillion software to keep a DE happy.
Have you actually tried that approach, with an individual user for all your programs?
In fact, I was attempting that kind of thing just this week, namely trying to run Spotify (distributed as a snap) as separate user in a visible window. Maybe it's possible, but it's certainly not trivial or turnkey.
That's what Android does. Desktop Linux should make this easier.