Very interesting and a cool project. I did briefly run a 300PDS on the same mount as you. I agree it was very large and it also hit one wall of my Obsy at a certain point, so I was never comfortable letting it work away at night. I went down to the 250, and found it much more usable, and the 1200mm FL wasnt much of an issue. You can see I also used weights, and have an ED80 in the background.
You might struggle with differential flexure using a guidescope - the tube is very thin and the mirror is very heavy. Also your mentioned than your guiding is now 3 x worse. We need to note that your imaging scale is now also 2.5 x finer, so you really need to be guiding even better than before. To get the most out of it, 0.45"/px RMS would be the target! I never managed it and subsequently sold it on.