To be honest, I followed your own instructions in this forum to create my first Raspberry Pi “DAW” … thanks for this tutorial by the way 
the sleep command was not the issue but I remember I had to tweak the Pd command and include the name of the audio interface directly in the command line, be as precise as possible regarding the buffer, the audio I/O etc… (start serialoscd, start Pd, wait for PD to start, give it a few seconds and start DSP, then make the ALSA connection for midi and audio) I understand you are using a custom “hat” for your sequencer, that’s why I can’t give a precise answer. I’m using an ES-8 on one Pi and another Pi lives in a Terminal Tedium. I’m sorry I can’t give a direct solution to your issue but yes, it might be a timing issue, or buffer, a very low buffer was causing freezing. What happens if you boot the Pi without Pd, wait a few seconds and load the script “manually” ?
