GATT is great if you want to make yet another heart rate monitor. I was building a lock that worked over BTLE and I spent most of my time trying to get around the problems with GATT. It's been 8 years now but I believe that that there was a bunch of extra overhead that bloated the packets and the packets were too small. Resulted in too much back and forth etc. I really just wanted a socket. iOS eventually allowed you to interact directly with the Logical Link Control and Adaptation Protocol aka L2CAP. Seems like every time someone try to design a generic structure that does one job but is also extendable, it's a mess. Just give me the byte stream and move on. L2CAP doesn't seem to be accessible on the pico yet, https://github.com/micropython/micropython-lib/issues/803