I have a similar problem with the Nano 33 IoT. According to it's specs it has BT4.2 which should support 512 bytes. As it is it only sends 20 bytes. I can't find a handle on how to tell it to break up the 512 bytes (or rather 120 bytes in case of my application) into chunks of 20 bytes.
In the utility\att.h I saw, that there is a function called setMaxMTU. Unfortunately I cdon't know how to make make it work. Simply changing the MTU size within the att.cpp didn't solve it for me.