Andy
I've used a similar combo (EQ6 + PHD + GSO RC8) and the guiding can be tricky! Some nights just a small amount of wind can cause huge problems. Also depending on where you are in the sky as well - some areas of the sky seem to guide better than others. And some nights guiding is terrible for no apparent reason - cable snags, balance, poor gear mesh etc etc.
There are some images on my site with this combo you can view here:
https://deepspaceplace.com/images.ph...&filter=GSO8RC
Most are on the EQ6 but some are on the PME - it's noted on each image. All with the stock focuser. A flattener might be a good investment for you.
My advice for PHP and collimation is to keep doing what you are doing - stick at it. Make small adjustments, make notes, check, measure etc and you will dial in the best results possible within the limits of your gear and all the other factors. I think this focal length is pushing the EQ6 to the max IMHO.
Also this might help
https://deepspaceplace.com/gso8rccollimate.php
Cheers
James