Opensource

I'd probably recommend something with GrapheneOS as it's pretty easy to install and use. I personally use it as well and never had any issues with it. You might want to look if your Banking apps work, but other than that, you can pretty much install most major apps through Aurora (like WhatsApp, Signal etc.)

If you want to go fully open, you could also try buying a Fairphone 4 and installing PostmarketOS on it, as I see PostmarketOS as the long-term ideal future OS for phones, because it frees us from the shackles of phone update cycles and gives the user full control over their OS (including changing the desktop environment).

But if you want to use PostmarketOS you have to be willing to tinker and not everything will work perfectly. Especially the camera will not be perfectly supported, even on the well supported Fairphone 4, your pictures will look worse than the ones you could take on Android, because there aren't good drivers and post processing yet. But it can be really cool if you are an enthusiast.