I did some more tests just with I2S passthrough and the decimation. The deciamtion works fine even when doing some Serial.print(samples,BIN); to check samples are coming are going.

The millisecond I enable the rf24.send(samples, len); to send the 48 byte packet the samples freeze up. New plan !

going to try and use https://github.com/greiman/FreeRTOS-Arduino

freeRTOS scheduler library. No idea what i'm doing with it but hoping I can assign the RF function send to one task and the samples IO circular buffer and I2S some others !