I have a 10" version of your scope and I have seen what you are seeing a number of times.
A laser simply isn't accurate enough for F4. I have a Hotech which is fine for F5, but not F4.
For this scope, I would use a Catseye collimation triple pack set.
Center the secondary mirror holder - make sure the vanes are tight. Use a small ruler.
Replace the useless secondary screws with Bob's knobs, make sure you get the long versions.
I would add two circles of plastic milk bottle between the screws and the secondary (create a hole for the center screw). It allows you to rotate the secondary and avoid the pits you will dig in the back of the secondary stalk. (Hard bolts into alloy).
Use the telecat to center the secondary within the focuser.
(A sheet of one colour paper behind the secondary and another across the tube makes this easy to see).
These two videos will help.
https://www.youtube.com/watch?v=wcvHOHwg1j4&t=2s
https://www.youtube.com/watch?v=3LbR1nIx-jw
Respot the primary accurately with a yellow hotspot. (who doesn't like a radiation symbol).
Use the blackcat to align you primary.
Infinity to align your secondary. Both primary and then offset.
Iterate between the two steps.
The bible for the Catseye collimation process:
https://www.youtube.com/watch?v=Vj12cx3tnsM
You will likely find the factory focuser suspect for imaging. I ended up with a Moonlite 2.5". The cool kids are going Litecrawlers now. (Sit down when your pricing).
We haven't discussed tilt yet. Depending on your coma corrector this can be finicky. I'm now in the GPU camp.
At F4, your dealing with sub millimeter tolerances. You will get there.
A recent test image of mine to show it is possible. (I'm still playing with tilt).
http://www.iceinspace.com.au/forum/s...d.php?t=164860