Earlier quoted context omitted.
BBB requires non-free drivers for the GPU, which are a huge PITA even if you don't mind them being non-free. You would end up with more-or-less a Nokia N9 clone. It's better to use something like i.MX 8M Quad - you may want to take a look at the Librem 5; or if you want something lower-end than that then there's also the PinePhone which is based on A64.
i.MX still needs binary blobs. I think the only ARM SBC today that can run blob-free today is Radxa's Rock pi 4.
With that said, I worry an rk3399 would run hot in a phone. Hopefully the thermals on the rk3588 are better. But rockchip is dedicating die space to a poorly supported "neural processing unit" now, so we will have to see.