Quite a late email cause I forgot to give a status update on the server
After intensive testing, we managed to get SID to install on the pSeries 550 - What we had to do was obtain the last Debian SID ppc64 netinst which still included Yaboot as a bootloader "during install" - this ended up being the May 20th 2019 ISO and we ended up still using a USB to avoid wasting too many CD's, we then had to load into expert mode and choose to install Yaboot as a bootloader instead of GRUB (we decided to use an all in / ext3 partition for best compatibility) - once installation was completed, everything worked as expected
So the important factors we came to conclude was that;
the pSeries (550 but maybe other models too) cannot boot from GRUB, potentially a firmware issue but not confirmed as the system log reports no errors/warnings
Yaboot has to be installed by the Debian installer itself, manual installation of Yaboot (chroot/rescue mode) will result in a broken Yaboot which is unable to load it's own configuration file (Results in issue stated when we first asked for help - Unsure if it's yabootconfig or yaboot itself because in both cases the system still can't detect the .conf and neither process throws any anomalies in the logs during yabootconfig/installing to PreP)
We would like to thank everyone for their help and suggestions and I hope that our discoveries help in anyway
If you'd like to know more then don't hesitate to ask
Deinsti and Daniel